well.. uhm.. it's obvious that you can find lots of good information and get help on good forums? so thats a reason
we can learn lots of useful things everyday.and also get helpful information. this is reason for big forum success. Abhilash.
Most successful forums were one of the first of their type and/or were under moderation by people whom had/have respect in their field.
I think having an acceptable amount of members, with interesting topics is a good trait to have. If someone comes to a Forum and realizes that there are few members and the topics are slow, then they probably won't stay very long. Also as someone else said, I think good Moderators makes it good too.