Most movie makers are in it for a quick, short-term income, whereas if you look at successful sci-fi, it tends to start out with a small, almost elitist audience, and over the course of years, gains a cult status amongst a much larger audience. This doesn't chime well with the quick-buck mentality, so they are constantly tinkering with perfectly good sci-fi in order to make it 'more accessible', and usually ruining it in the process.
Oddly enough, the audience the Studios demand the screenwriters write for doesn't actually exist, and it tends to alter films in precisely the way that would piss off the actual audience the most.
Edit : For example, the remake/relaunch was a good popcorn movie, but, dear god, the science....