I have wondered this too. Came up with a few catagories from the responses I have gotten.
Forced - Although they do not believe it, either through parents, spouses or family members. They are forced to go. Perhaps a situation of, 'if you are in my house and under my roof, you will do this.' Either way, the 'forced group' is very common and I think many of us have been there at some time or another.
Faders - Those who have questioned the religion, found the problems, but are not yet 100% convinced that the religion is all wrong or myth. Perhaps they still have that thought stuck in their mind of, 'I know so much about the Bible (as instructed by the Watchtower) that no other religion will ever seem right. So they just stay until something comes up to make them more disconnected.
Lazy - They do not believe it, do not see anything that makes it real and frankly they think it is full of crap. However, the idea of actually leaving and making a stand is just too much work. So they go, and just take the easy way out.
Savers - Believe that despite them knowing it is all crap, all cultlike, etc. They think that they should still stay connected, still read the publications and attend, so that they can save those who need help in exit. These are the ones who take all the publications apart, peice by peice, and use names and photos on here that are not them. That way they can still 'save' those who need it, without having to exit themselves or be exposed.
Sick - People who just plain like watching others in a hell hole and yet do not see the insanity of being their with them, themselves.
I am sure you think of other groups. However, this pretty much sums up all the ones I have run across. They always fit in their some way or another. Hope that answers your question.