I stayed so long because I wanted to. For a long time, my fear of the unknown was greater than my misery. And part of that unknown had to do with protecting our children once their dad and I split, a battle I'm still fighting 6 mos. after leaving.
Once I got away with friends for a few weeks, I was able to see the futility of trying to get someone to change who didn't (and doesn't) want to change. That distance and perspective allowed me to recognize that it was time to move on, despite my fears.