As established this season, Echo still has those memories, so she knows her way around. Basically, I think its Paul and the promise that he will help her find Caroline that's keepin' her in the Dollhouse, since she can pretty much leave at any time, as Echo.
Well, we just found out that she's going to need to get that GPS removed. I assume Dr. Saunders was able to remove her GPS by herself, but Echo doesn't have that knowledge base.