There are lots of things going on, but I think one of the biggest hurdles is just plain fear of the unknown. You're USED to a certain level of abuse in your life--it probably feels normal by now, so any time the level goes down for a bit, or the abuser starts being nice, and attentive, I think you start thinking that staying is the path of least resistance. It feels easier to continue to endure what you've been enduring than to rock the boat by leaving.
That's why working with an advocate who can provide you with support and encouragement, as well as a safety plan, can be very helpful when you're trying to extricate yourself from an abusive relationship.
Because it isn't going to STAY OK. Sooner or later, he'll be behaving exactly as he always has.