Why is it more difficult for a Mormon woman to control whom she is eternally sealed to?

Simple answer: that is how/what God commanded the people to do. The exact reasoning of polygamy being instituted was unknown, except the reasoning given in Jacob (to raise seed unto [the Lord])^{1, 2}. Plural marriage is an acceptable practice only when the Lord commands it³.

Jacob 2:27-30

27 Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none;

28 For I, the Lord God, delight in the chastity of women. And who*edoms are an abomination before me; thus saith the Lord of Hosts.

29 Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes.

30 For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things.

Going with this principle it can be conjectured that because a child takes 9 months to grow, one man with multiple wives can raise more seed than one woman and multiple husbands. This does not seem to be stated explicitly in these terms anywhere in LDS canon and is my conclusion.

_{1 Plural Marriage and Families in Early Utah}

_{2 Doctrine and Covenants 132:34}

_{3 Foundations of the Restoration Manual-Plural Marriage}

The practice when dealing with deceased persons is that a woman may be sealed to any or all husbands to whom she was legally married, since ultimate determinations of worthiness or preference must be necessarily be made beyond the veil, in the Spirit world. Members are encouraged to trust in the Lord's goodness and mercy, to make whatever adjustments will result in the greatest happiness for the people involved.

USA Today is presenting correct facts, but they're a bit outside of context, which has allowed you to jump to a conclusion that's actually quite wrong.

As Mikeazo pointed out, the Lord will not force anyone to be married to anyone. That would be unjust.

But you appear to believe that temple sealings are as disposable as civil marriages. They're not. You also appear to believe that a woman who marries after her husband has passed away is doing so because she wants to be married to two men for time and all eternity or because she doesn't want the first husband anymore, preferring the second. Those are false assumptions.

The rules of marriage in the LDS Church are based on Doctrine & Covenants 132—which also is the source of the Church's rules regarding plural marriage (popularly known as "polygamy"). Basically, the rules of marriage and polygamy allow a man to marry multiple women—but not a woman to marry multiple men.¹ Therefore, it is permissible once a wife dies for her husband to marry and be sealed to another wife. But it makes no sense that a wife, when her first husband dies, be sealed to another husband when the second marriage is for companionship and not eternal exaltation.²

I hope you caught that last bit—not for eternal exaltation. Part of the problem here is that we're convoluting two ideas: obedience to the commandment of the Lord in D&C 132 to attain exaltation in the Kingdom of our Heavenly Father, and earthly marriage for companionship.

Therfore it's important that you understand the following: in the LDS Church men and women both have the same rights and privileges when it comes to terminating a temple sealing. Men do not have more rights than women in this regard.

But when it comes to maintaining the rights and privileges associated with a temple sealing, that maintenance is dictated by the commandment of the Lord in D&C 132, which permits men to be sealed to multiple women but not vice versa.

Please note that the value of Mikeazo's answer cannot be overstated, and I've upvoted it. The Lord will not force anyone to be married to anyone they don't want.

One more thing, and this isn't really part of your question but as the question touched on plural marriage it's worth saying as a follow-up detail. I said D&C 132 contains the basic rules for marriage in the LDS Church, but there is one exception. Official Declaration 1 discontinued the practice of cohabitation. Said more simply, men are no longer allowed to be simultaneously married³ to more than one woman. So while all the rules of plural marriage are in play, the factual reality of plural marriage does not exist. I've heard one man crow that he was plurally married because he had a civil divorce from his first wife, but not a sealing termination, and then subsequently was sealed to a second wife. On paper this looks like polygamy—but it's not. D&C 132 states that a temple sealing mus be confirmed by the Holy Spirit of Promise, and since we believe in sustaining the law (Article of Faith #12), where there's no civil marriage, there's no temple sealing no matter what the paperwork says. Chalk it up to the complexity of bureaucracy.

_{¹ I say "basically" because it was not uncommon in early Church history for women to also be sealed to Joseph Smith or other prominent Church leaders. This was based on D&C 132:41. I don't recall if any such union was sexually intimate and I believe the practice ended before the 1880s. Anyway, this is why the Church's practice is referred to as "polygamy" and not just "polygyny."}

_{² Contrast that with this: a woman previously married civilly who either divorces or is widowed and subsequently marries a second man can be sealed the second man for time and all eternity. In fact, that woman can choose which man to be sealed to.}

_{³ Married—not sealed—as in "sleeping together."}

The information I am presenting comes from the newly released General Handbook of the Church of Jesus Christ of Latter-day Saints.

I think the most important section in that handbook for answering your questions comes from Section 38.5 (emphasis is my own)

Members who have concerns about the eternal nature of the sealing ordinance and their associated family and spousal relationships can find peace in the knowledge that Heavenly Father is loving and just. Faithful children who are sealed to parents or born in the covenant retain the blessing of eternal parentage. This is so even if their parents cancel their marriage sealing, have their Church membership withdrawn, or resign their membership.

Members who are divorced but still sealed to the former spouse are often troubled by the thought of that sealing. The sealing will not be compulsory in the postmortal life for either a man or a woman. If temple covenants are broken and no repentance is made, the sealing between the husband and wife is revoked. However, those who keep their covenants will retain the individual blessings provided by the sealing. This is the case even if the spouse has broken the covenants or withdrawn from the marriage.

Given what is said there, I don't think the premise of your question is really correct, that it is more difficult for female members of the Church of Jesus Christ of Latter-day Saints to "control" who they are eternally sealed to. Definitely, as detailed in Section 38.5.1.2 of the handbook, there are differences in practices of sealings after divorce and after a spouses death (as detailed in 38.5.1.3) for men and women. Those sections are for living spouses, see 38.5.1.7 for the cases when both spouses are deceased. The exact reason for those differences is, as far as I know, not explicitly stated in the handbook or Church canon.