According to soul sleep adherents, why would God allow people to be massively misled by the parable of the rich man and Lazarus (Lk 16:19-31)?

Why would God allow people to be misled by an ambiguous verse of scripture?

God's people must have free will, they must exercise that free will based on knowledge, and that knowledge must be based on an understanding of the scriptures.

Those that pick verses out of context and use them to prove what they already believe (eisegesis) are not gaining true knowledge or right understanding. They are shaping their religion to match their beliefs.

This very process separates the sheep from the goats, the wheat from the chaff.

[The Jews in Berea] were more fair-minded than those in Thessalonica, in that they received the word with all readiness, and searched the Scriptures daily to find out whether these things were so. Therefore many of them believed, and also not a few of the Greeks, prominent women as well as men.
— Acts 17:11–12

These people were open minded and worked hard to verify what they were told; they didn't simply accept it. And when they did understand it, they accepted it and truly believed. They were shaping their beliefs to match the truth.

Whom will he teach knowledge? And whom will he make to understand the message? Those just weaned from milk? Those just drawn from the breasts? For precept must be upon precept, precept upon precept, Line upon line, line upon line, Here a little, there a little.
— Isaiah 28:9–10

God's truth isn't presented wrapped up in a neat little package. Instead, the scriptures have information scattered throughout them, and much of that information is ambiguous.

Those people that are willing to research and study, to take into account the various contexts that each verse is affected by, and to compare many scriptures in order to resolve the ambiguities (exegesis) will discover the consistent truth. And that consistency will reinforce their belief.

Therefore, since we have this ministry, as we have received mercy, we do not lose heart. But we have renounced the hidden things of shame, not walking in craftiness nor handling the word of God deceitfully, but by manifestation of the truth commending ourselves to every man’s conscience in the sight of God. But even if our gospel is veiled, it is veiled to those who are perishing, whose minds the god of this age has blinded, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them.
— 2 Corinthians 4:1–4

God has allowed people to be massively misled for six thousand years. During that time, only a very small percentage of mankind has been called to receive God's truth. It is they that will rise to meet Christ at his return, and they that will help teach the rest of humanity in the Kingdom of God.

Yes, Luke 16 does talk as if the dead are conscious.

Some people will simply accept this at face value, because it fits so nicely with what they already believe and what their friends and families already believe. They have freely chosen to be misled in the name of convenience.

Others will notice that the concept of awareness while dead contradicts many scriptures that say the exact opposite. Knowing that scripture is self-consistent, they will reason that either:

• This one scripture should be taken at face value while the many others must be rationalized away.
• The many scriptures are self-consistent so this one scripture must obviously still be misunderstood.

The "fair-minded" people will take the latter position, will further research the context, and will finally realize that Jesus's story was simply using a popular "meme" of his time to make a point.

These are God's people, not those that simply act as if they are (even though they may be truly sincere about it).

Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of My Father in heaven. Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
— Matthew 7:21–23