Plus, for a lot of people, diet is the hardest part and often the last thing to fall into place.... Lots of people have no problem working out, etc but just can't get their diet in line...
coming to your original question... the last time I checked the 3 most popular boards where (in no special order):
1. Anabolics: That is why this is the MOST controversial site on the Net
2. Chat and Conversation: Because we love drama and talking shit nonstop (as long as we are not training)
3. Naked Women: Because we have Perkele with his apparently unlimited collection of pics...
Besides, most people tend to believe "they have their diet in check"... only when they come here they realize it is not the case and then they come and check for a while.