(a) With circular polarisation and the like, channel separation is about as good as it's going to get for the foreseeable future and stereoscopic imaging doesn't interfere with colour any more, so few people object. (Previous attempts sacrificed colour for depth perception.) (b) It's harder to video in the cinema. (c) They can (eventually) re-sell everyone their whole film libraries again, but this time in "glorious" stereo!