The main theories I understand to pertain to George's commitment to changing the movies and not releasing the UOT are:

1. His vision is changing constantly.

2. Changing the movies renews the copyright.

3. He doesn't owe as many royalties to other producers/writers/editors/etc. if he only releases the "Special Editions."

4. Bitterness over his divorce led him to change the movies and diminish Marcia's presence in them.

I don't know how these all fit together, but it's a combination of things. It seems that during the build of Skywalker Ranch George's priorities changed. That affected his marriage, and it's all presumption after that.