I was wondering this myself

MMA has a habit of quickly disgarding any fighting style or technique that is not effective. Kung Fu and Karate have not survived as a realistic form of fighting.

I think Bruce Lee would have lasted ok at the very beggining of the UFC but fall short when he was up against a Royce Gracie or a Ken Shamrock that would grab him and pull him to the mat.

This isn't to say he wouldn't adapt. He had an unquenchable urge to learn fighting techniques and would have loved the MMA era.