Question: Why was Canaan cursed?
Response: The first thing to note in this regard is that Canaan, Ham's son, was doubtless a mere infant when Noah pronounced this curse (and that is only if we assume, as I do, that Canaan was the eldest son, and only listed fourth in the genealogical order in Gen.10:6 and 1Chron.1:8 because of his accursed status). For only Noah and his wife and his sons and their wives, eight in all, entered into the ark (Gen.7:13; 1Pet.3:20). So whatever the basis for the curse, it is clear that it was not based upon anything that Canaan had already done. Ham, on the other hand, had, by any account, just acted in a perverse and disrespectful way towards his father (cf. Ex.20:12; Eph.6:2). That is true however we may wish to interpret what it was that he "did" to Noah (Gen.9:24: Hebrew 'asah).
The fact that this curse applies not to Ham but to one of his offspring thus makes it certain that this is a prophetic curse. Blessing and cursing when uttered by prophets of God (which Noah was) are truly "prophetic". That is to say, they are given through the prophet but the content really belongs to God, for prophets "speak forth" for God about future events (this is the etymology of the Greek word prophetes). So, for example, when Isaac blessed Jacob and cursed Esau, even though he had originally been tricked, the blessing and cursing was of God and so could not be reversed (Rom.9:10-13; cf. Mal.1:2-3). Thus blessings and cursing of this special prophetic type are both predictive and determinative, because they originate with God who both knows and decrees all things (Rom.8:28-30).
The cursing of Canaan represents God's prediction about him and his offspring, given through Noah, given at a critical time when all who would hear about it later like you and I could plainly see from what had just happened that a trend of the highly disrespectful and perverse behavior clearly present in Ham would continue in his offspring, the Canaanites (from whom, for example, come the inhabitants of Sodom and Gomorrah: cf. Gen.10:19). So the curse (a prophecy from God) merely predicts that Ham's descendants through Canaan will continue to be characterized by perverse behavior, since this is what Ham was like and what one particular branch of his descendants (who will reflect this behavioral trend) will be like (albeit from their own free will). The prophetic pronouncement demonstrates that all this has been sealed up in the decree of God, looking into the future with divine foreknowledge.
It is also important to note that the Genesis chapter nine "curse" has been used in the past to advance some despicable and completely un-Christian agendas. As Christians, we need to be aware that there will never be a shortage of those willing to "twist scripture to their own destruction" for the sake of some private or political purpose (2Pet.3:16). And it is clear that there is no basis whatsoever for assuming that this curse is still applicable today. For while the curse is responding to the behavior of Ham (and is thus sometimes called "the curse of Ham", a bit of a misnomer or at least misleading), it applies only to the line of Canaan, and the Canaanites have long since disappeared into the dust of history to such a complete degree that it would be ridiculous to assign even a small portion of their identity to any nation or group presently extent on earth.
All this is a long way of saying that Canaan was cursed in response to his father's perverse and disrespectful behavior, but that the curse itself is divinely based, and both predicts and decrees the future behavior of a particularly degenerate branch of the human family through Ham and Canaan, behavior that is biblically and historically verifiable, but which is also past tense in terms of its concentration in this one now defunct (or at least unknowably dispersed) genealogical strain.
Please also see:
"How could a loving God order the destruction of the Canaanites?".
Cursing by Association
The Generational Curse
On prophecy in general please see "Hermeneutic Issues" in CT 1, and also "Prophecy Questions".
Hope this gets to the heart of your question.
In Him who died for the sins of all mankind, our Lord and Savior Jesus Christ.