This question is for Biblical Unitarians. Why does the "Shema" at Deuteronomy 6:4 use the Hebrew word "echad" instead of the Hebrew word, "yachid?"

There's an extended discussion of this at the REV's entry on Deuteronomy 6:4.

In particular, they quote Anthony Buzzard (The Doctrine of the Trinity: Christianity’s Self-Inflicted Wound, p. 25), who says

“It is untrue to say that the Hebrew word echad (one) in Deut. 6:4 points to a compound unity. A recent defense of the Trinity argues that when “one” modifies a collective noun like “bunch” or “herd,” a plurality is implied in echad. The argument is fallacious. The sense of plurality is derived from the collective noun (herd, etc.), not from the word “one.” Echad in Hebrew is the numeral “one.” “Abraham was one [echad]” (Ezek. 33:24; “only one man,” NIV). Isaiah 51:2 also describes Abraham as “one” (echad; “alone,” KJV; “the only one,” NJBO, where there is no possible misunderstanding about the meaning of this simple word.”

As the commentary goes on to note, 'echad' here is intended to convey Yahweh is 'alone' God - there are no others.

Also, the context of the Shema in the Old Testament and where it is quoted in the New Testament indicate that the Shema is not saying “God is ‘one,’” but rather is saying that Yahweh “alone” is God. The context of Deuteronomy 6:4 is using the Hebrew word echad (#0259 אֶחָד; “one, only, alone”) in the primary sense of “only” or “alone,” in contrast to the number “one,” and Mark 12:28-34 confirms this. Note how Deuteronomy 6:4-5 flow together and thus make a major—and logical—point: “Yahweh is our God, Yahweh alone! And you must love Yahweh your God with all your heart, all your soul, and all your strength.” It is because Yahweh “alone” is God that we can worship him with “all” our heart, “all” our soul, and “all” our might. If we had more than one God, our worship would have to be divided between all the gods we served, and each god would get only “part” of our heart, soul, and strength.

So the answer to the question "Why echad instead of yachid?" is that echad fits the purpose. Why wouldn't it be used, when it adequately conveyed what was intended - that Jews should shun polytheism and instead have Yahweh alone as the Israelites' God? As the linked article says

"[T]he Old Testament was given by God to the Jews so they could know and obey Him, and never in the more than 3500 years since the Shema was written have the Jews understood it to refer to a compound unity in God—quite the opposite. They took it to mean that there was only one God, and they fiercely fought against polytheism throughout their history. So if the Shema was God’s attempt to reveal a compound unity in Himself, the attempt was an epic failure. It makes much more sense that God gave the verse to the Jews and intended it to mean what the Jews say it means. Furthermore, the Jews did not take the Shema as their primary statement of monotheism because many other verses in Hebrew Scripture made that point just as clearly."

As the article "Echad" in the Shema points out

"the standard Hebrew word to denote joining, unity or togetherness is yachad, not echad."

They go on to say

"Given the theme of YHVH's centrality in Deuteronomy (see below), and given the command aspect of the Shema ("and you shall love YHVH your God"), the sense [for echad, amongst its various senses] of uniqueness seems most appropriate in this verse."

The Jewish Publication Society's translation of this verse, quoted in the article, comports with this point.

"Hear, O Israel! The LORD is our God, the LORD alone."

Making a similar point to the REV commentary, the article continues

"Thus, in the whole expanse of Israelite history, the Shema declares YHVH's singular uniqueness as Israel's God. She was to listen (shama) to that declaration and obey (shama) its implications, and love the One who chose to reveal himself to her."

And finally summarizes the situation similarly,

"Summary: There is no internal evidence in the Hebrew Bible that any generation of Israelites understood the Shema as a reference to a compound unity in the Godhead."

What "אֶחָֽד" (Echad) Means

The Hebrew word "אֶחָֽד" (echad/ehad) is used in hundreds of places throughout the Old Testament. The first is Genesis 1:5 where it refers to "Day 1" of creation. The word "echad" is the literal number "1" in Hebrew, spelled out (like English "one"). Notice the leading "aleph" in its spelling. Hebrew numbers follow the consonants in its "alephbet" (alphabet), beginning with the "aleph" as 1, and continuing. They have full spellings for their words as well, but just the "aleph" (א) by itself would be the numerical digit in Hebrew for the cardinal number "1".

Depending on the context, "echad" might also be used as an ordinal number to denote "first". So in Genesis 1:5, it could be understood also as referencing the "first" day. No scholar would try to say that "echad" in Genesis 1 referred to a "unity."

Another example of the use of "one" (echad) in Genesis would be the following:

And unto Eber were born two sons: the name of one was Peleg; for in his days was the earth divided; and his brother's name was Joktan. (Genesis 10:25, KJV)

Again, it would make no sense to claim that "Peleg" was a "unity" of Eber's sons. The word clearly means "one" in a cardinal sense.

But let's take a closer look at the typical translation of the shema, as its unusual wording seems to be the real source of this question.

The Shema

"Shema" means "listen" or "hear" with an emphasis on paying attention. It is the first word of this special passage which Jews faithfully commit to memory--a passage that extends well beyond its first verse, but we will focus on just Deuteronomy 6:4.

In English, it may be translated like this:

Hear, O Israel: The LORD our God is one LORD: (Deuteronomy 6:4, KJV)

But, as the questioner points out, that wording is a bit unusual. In fact, the Hebrew itself may be seen as unusual in that several nouns are placed together without any intervening words, leaving their precise translations slightly ambiguous because no punctuation, e.g. the comma, was used in Hebrew writing at that time.

Consider:

שְׁמַ֖ע יִשְׂרָאֵ֑ל יְהוָ֥ה אֱלֹהֵ֖ינוּ יְהוָ֥ה אֶחָֽד׃

Transliterated, this might be: "shema Yisrael Yahweh elohenu Yahweh ehad."

The first word is a verb: "shema." "Listen!" The rest of the words are all nouns, except that "elohim" has a pronominal suffix for "our" and "ehad" may represent an adjective (grammatically, a number like this could be either a noun or an adjective).

Hebrew Construct Chains

A review of Hebrew nouns in their construct state is important at this point. Two nouns together which have equal definiteness, either both being definite or both being non-definite, form a construct chain; they are placed together in a genitive relationship, usually requiring something like the preposition "of" between them in English.

But what makes a noun "definite"?

Hebrew nouns will be definite for more than just having the definite article attached. It can be definite for any of these reasons:

1. It is a name.
2. It has the definite article.
3. It has a pronominal suffix.

If the consecutive nouns are unequal in definiteness, they are not in construct state, but have a predicate adjective relationship, meaning there must be a verb of being linking them.

Translating the Shema

To translate the shema, our first task is to identify the words which are definite, and to distinguish them from those which are not.

The last two in the sequence have unequal definiteness. Between these, therefore, we must insert a verb of being, e.g. "is."

Let's try what we have so far.

Listen Israel Yahweh our God Yahweh is one.

Do you see the problem? Yes, it's the lack of punctuation. The Hebrew words here are not fully amenable to translation as coming from construct state, requiring the addition of "of." What it seems to need instead is punctuation. And just here is where translations mostly differ.

It could be one of these:

Listen, Israel! Yahweh, our God Yahweh, is one.
Listen! Israel of Yahweh, our God; Yahweh is one.
Listen, Israel of Yahweh! Our God, Yahweh, is one.

Perhaps none of these alternate translations would settle any questions a Trinitarian might seek to insert here, but they might, at least, make better sense than the KJV rendering or that of some other versions. Jews to this day will consistently understand the Shema as saying that we have but one God, and that is Yahweh (for whom they will say "Adonai" or even "Hashem" instead of pronouncing the name). No true Jew (Jewish by religion) would even consider Trinitarianism as a possibility.

What Jews Interpret: Elohim vs Trinity

Here is an interesting anecdote from a non-trinitarian written over 150 years ago that mentions what the Jews believed about the Trinity (written by the Christian on the topic of the Trinity):

Its origin is pagan and fabulous. Instead of pointing us to scripture for proof of the trinity, we are pointed to the trident of the Persians, with the assertion that “by this they designed to teach the idea of a trinity, and if they had the doctrine of the trinity, they must have received it by tradition from the people of God. But this is all assumed, for it is certain that the Jewish church held to no such doctrine. Says Mr. Summerbell, “A friend of mine who was present in a New York synagogue, asked the Rabbi for an explanation of the word ’elohim’. A Trinitarian clergyman who stood by, replied, ‘Why, that has reference to the three persons in the Trinity,’ when a Jew stepped forward and said he must not mention that word again, or they would have to compel him to leave the house; for it was not permitted to mention the name of any strange god in the synagogue.’ Milman says the idea of the Trident is fabulous. (J. Loughborough, November 5, 1861, p. 184.9)

Conclusion

Anyone who questions the reason for a particular wording in the Scriptures has one of two things in mind: 1) to understand better the reason for the word choice given; or 2) to question that word choice as if his or her wisdom were superior and it could have been said better in some other way.

Let's not be in the second category. The Bible is plain enough: "echad" means "one"--a number that is consistently used throughout the Old Testament as a cardinal number, with minor usages as an ordinal or that are translated in some other manner ("a/an", "any", "another", "other", etc.).

More detail on the Hebrew usage of "echad" can be found on an interesting website HERE.