I don't think there is any legal shenanegans going on. The truth is, she just wants out of the spotlight. She gave up editing to raise a family long before she divorced George Lucas. It was a decision she made around 1979 or so. There's almost certainly nothing to do with Star Wars copyrights and Special Editions and the like.

I have a feeling that by the time she had gotten her family time (the reason she left the biz and then Lucas) it was the 1990s, like Lucas, but unlike Lucas she couldn't really return to the industry if she wanted to because she hadn't edited anything since the late 1970s and probably couldn't get back into the union/ everyone forgot about her. She would have been about 50 years old keep in mind. I think by that time she had gotten involved in other ventures and didn't really have the inclination to try to break back in.

@ the prequelsrule: Marcia came to me after I wrote the only existing biographical article on her and her work. Our correspondence was not too extensive and she hasn't contacted me since.