The classes contested in the Olympics are as outlined by the International Canoe Federation's official rules. Interesting to note, although OC may not soon see the light of official international competition, womens canoe soon will become a reality. Now the races will include C-1W and C-2W. Ladies, it is time to step it up.
It might in part be that the difficulty of the courses would probably fill open boats quickly and it would be very difficult to finish the course. Class IV slalom is not easy full of water, I would assume.