We are not told why. After all, Ham did have other sons, and in just a few verses (Gen 10:6), we will see in the genealogy that (apparently) Canaan was the youngest; though perhaps he was listed last due to his shame. In any event, we are forced to conclude that there was some special reason, known to Noah but unknown to us. Since the curse is generational, it might very well be incest (or be some lesser perversion that develops into incest), a thing known to run in families. Moreover, in Ex 18, God forbids the entering Israelites to behave “after the doings of the land of Canaan, whither I bring you” (Ex 18:3), and he then rattles off a long and disgusting list of varieties of incest, child rape, child sacrifice, etc., and concludes the list of horrors by saying, “Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you” (Ex 18:24). Henry aptly points out that in saying this, Moses was partly justifying for the sake of the Israelites the justice to be meted out to the Canaanites—the justice which God was presently ordering, and which the Israelites were to administer.