Discs are too heavy. This level of braking is unnecessary unless you are doing muddy and steep cyclocross racing, or loaded touring in the rain.

For some reason I am getting euro UCI 'cross races in my cable package. This is almost as bad as watching golf, but I did manage to suffer through one race to the end.

Everyone is using cantis. Everyone. Except the last lap, where a rider pulls out a disc-brake equipped bike out of the pit, crosses the line for the win, and raises this fresh bike over his head for the victory photo-op. So I assume discs are good for sponsorship obligations and new bike marketing.