I don't have an immediate answer to this question, but it is interesting that Ham is specifically referred to as "the father of Canaan" in the verse that describes him seeing Noah's nakedness. There must be some particular reason why that is. Ham had four sons. Genesis 10:6 tells us that Ham's four sons were (presumably in order of birth, I think that's how Biblical genealogies normally work) Cush, Mizraim, Phut and Canaan. So Canaan was the baby of Ham's family. But he is singled out for special mention. Genesis 9:18 is rather interesting in this respect too:
And the sons of Noah, that went forth of the ark, were Shem, and Ham, and Japheth: and Ham is the father of Canaan.
So even though Ham had those four sons, there is great emphasis on him being the father of Canaan. And none of the other three sons were cursed by Noah - only Canaan. Kenneth may be right about God having already blessed Ham, but then why would Noah curse Canaan in particular, and not one or even all of Ham's other sons?
Having said that, maybe there's a reason why God didn't explain this in greater detail. There are still some salutary lessons to be taken from this passage, in particular about the dangers of drunkenness and how it can have a lasting negative impact on a family. Perhaps one purpose was simply to explain why Canaan was a corrupt nation from the outset. Also, in addition to being a curse, it was a prophecy, because Canaan ended up being conquered by Israel when God judged it for its sins. It is perhaps appropriate that the curse was based on some sort of lewd conduct, because Canaan became one of the most morally depraved nations, if not THE most depraved, in the ancient world. After all, Sodom and Gomorrah were located there. Even by the standards of the present day, Canaan was incredibly wicked. As evil as the world is right now, there are still some sexual sins (like incest and bestiality) that are frowned upon. No doubt there are people committing them, but they're still considered crimes. Unlike homosexuality, which is "socially acceptable", most people still find incest and bestiality abhorrent (and rightly so). But Canaan embraced them. Leviticus 18 gives us a long list of forbidden sexual practices (also outlawing child sacrifice), and just before that, God singles out Canaan for special mention:
After the doings of the land of Egypt, wherein ye dwelt, shall ye not do: and after the doings of the land of Canaan, whither I bring you, shall ye not do: neither shall ye walk in their ordinances. (Leviticus 18:3)
Then in Verse 24 of that chapter, we read:
Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you
So everything that was prohibited in Leviticus 18, Canaan was doing. And all that corruption and defilement began with the incident of Noah's drunkenness and subsequent curse on Canaan. That was the root of it all. And perhaps that's the main point. After all, Genesis is the book of origins, and this is the origin of Canaan being such a wicked nation.