There is an irony to the plainness of this verse. It is a verse that plainly teaches things that we plainly don't believe.
It says that the prophets taught the truth plainly. The Jews, meaning ancient Israel, despised the plain teachings of the prophets.
What plain teachings? The ones that the wicked always despise: repentance and commandments. Those are the ones the prophets get killed over. They are the teachings that make the wicked feel "judged".
Because they despised the plain teachings, they sought for things that they could not understand. That is nothing new, incidentally. It is a not uncommon that the wicked replace plain teachings to keep the commandments with mysteries, with wizards that peep and mutter, or with Gods that are three in one and are everywhere and nowhere.
What was God's response to all this? Well, among the Jews God took away his plainness from them. He gave them prophecies that were difficult to understand, or that could be easily misconstrued. He made the scriptures somewhat difficult to understand.
Well, the verse itself is plain enough.
For two reasons: God did it because they desired it, and he did it that they may stumble.
And that last part is the part we plainly don't believe. We have Christ's parable which conceal their meaning. We have Isaiah's prophecies. We have Christ teaching his followers that they must eat of his flesh and drink his blood leaving them reeling in confusion in John 6. We have the revelation of John.
And honestly, if we just ask ourselves when we really come to understand a verse in the doctrine and covenants the question: "It took me years to understand what that verse was really getting at, and it was God himself speaking it, could he not have worded it so that the meaning was perfectly plain the first time I read it?"
The answer is that yes, he could have worded it so that his meaning was perfectly plain. But why has he not done so.
And the answer is that it is not because it is a better way to reveal and uncover the truth. Rather it is a way to conceal the truth. We read of Christ's parables in the scriptures and how the disciples didn't understand them at first, and don't recognize the same thing look back at us out of the text of many verses in the doctrine and covenants or even the Book of Mormon staring straight at us out of the text.
We read a verse today, that we have been reading for twenty years, and we finally understand it. And still don't see that the scriptures are not written solely to reveal, but also, and quite intentionally, to conceal. It is not just that God reveals things line upon line. It is also that he conceals them intentionally so that we understand them only when we have payed the due price in asking, seeking, and knocking according to the commandments.
And, as he plainly teaches, it is also done so that the wicked may stumble. Yes, the Jews did reject the words of plainness and seek out that which they could not understand. But they are not exactly the only people to do so.
Why did Joseph Smith find so many churches in his day? Was it just that some verses were altered or missing from the King James version.
Hardly. You could pass out the JST in place of the KJV without causing any marked change in the number of denominations to be found on the earth. The scriptures are frequently ambiguous or difficult to understand.
Because God hath done it so that they may stumble.
Now this runs wildly contrary to some of the notions about God which we have obtained by thinking like psychiatry would have us think. It runs wildly contrary to some of our notions of just what God would and would not do.
But then, it is rather plainly stated in the scripture. So we will have to choose, whether we prefer to believe a scripture that plainly says "God hath done it, that they may stumble", or whether we will despise that plainness, preferring something more appealing in its place.