I believe Jaycee Dugard confirmed the perp in her case never molested again after the birth of her second child. Found God, apparently. Didn't release her or her kids though.
I was reading about this, one of the criminologists said that although there had never been a survey done asking them, there were a few ideas based on specific examples. Sometimes they think they become afraid of getting caught, or have a close call. Some find other outlets for their issues. And they think for some it's age, either they are no longer physically robust or they become less driven by whatever makes them want to do it and perhaps more stable. There is some thought that hormonal changes might be part of it.