I've asked this same question of attendings and graduating residents. The answer I get most often is that people enjoy the low stress, "all included" hospital packages with minimum of paperwork (residents do it), guaranteed predictable decent paycheck, included insurance, and consistent hours.
I hear, whether it's correct or not, that working in a hospital (even though for a lower salary) can be more satisfying in that you can concentrate on patients and not on the "business," and the headaches that come with running one. It's just not worth it to some people to try and make the jump from a guaranteed $115,000 with bonuses to the potential $150,000 -$400,000+ that can be had in private practice.