1. No one person controls Star Trek since Gene Roddenberry died. Star Wars is at the mercy of George Lucas. This makes it harder for someone to decide to start 'improving' things.
2. Star Trek has many more shows, movies, etc, to earn them royalties on. Lucas has a large company and only a handful of movies to support it. In addition, Star Trek seemed to focus their creative talents on creating new shows, Star Wars only had the OOT for a long time.
3. Star Trek doesn't seem to suffer as much from "my original vision" ideas. You didn't have Gene Roddenberry doing things like adding stuff to fit the "way he originally invisioned them".
Of course, saying that Star Trek gets better treatment overall, didn't see Enterprise or most of the Star Trek stuff they put out since Gene Roddenberry died. Maybe thats why they are doing the changes. Most things they have done recently failed badly, so they are going back and revisiting their older stuff.