It could be for several reasons — distribution of power being one of them. The wardens have complete control over the life support system of the facility and a revolt could easily end up being a catastrophy.
There's also the psychological aspect, a sort of... large-scale case of Stockholm's Syndrome. Mistaking lack of cruelty for kindness, coupled with a relatively comfortable environment that's easy to live in. We have everything we need, why should we bite the hand that feeds us? Additionally, there are quite a few prisoners that are better off here than they would be should they be sent home.
All in all... it's difficult to form a united front and even more so to find the right angle of attack.