...science fiction films -- like science fiction literature -- value the idea over the idea's delivery system. So if you deliver a 150-foot reptile who is the embodiment of the mid-20th Century fear of nuclear annihilation (like Gojira), or tap into the Baby Boomer terror of the death of its own beautiful youth (a la Logan's Run), you can get away with letting a lot of other stuff slide, like plot, acting and production design.
This is so obviously true that it's painful. It's a problem for sci-fi geeks, many of whom are willing to let slide fundamental problems with a movie as long as the central concept is exciting and interesting (people as food, eh?) Of course the willingness to sacrifice a little form for the sake of idea is no less true of written science fiction as well, but though science fiction books may be dominated by novels barely worth the paper they're written on, the "classics" of written science fiction include works by the likes of Frank Herbert and Gene Wolfe, works that obtain the high rank of "literature" beyond the genre of science fiction. The problem with science fiction movies is the problem of movies in general, that it's far easier to turn out crappy movies than it is to turn out thoughtful and challenging movies, whatever the genre. The peculiarities of science fiction make this an even bigger problem for movies, where lazy screenwriters, producers and directors can simply fit in more monsters, more lasers and more giant spaceships and American audiences will eat it up, just like the eat up all the other tripe that filmmakers turn out to get butts in the theater.
Of course, the situation isn't entirely hopeless. Despite this dynamic thoughtful filmmakers manage to turn out thoughtful films that are undoubtedly science fiction, like "Solaris" or "Blade Runner" or "2001." And some filmmakers manage to make good science fiction movies even if they are filled with monsters, like "Alien" or giant spaceships and lasers, like "Star Trek II: The Wrath of Khan" (by far the best Star Trek movie ever, and an all around great movie.) So there's hope, as long as you're willing to wait every few years or so for one of these quality films to sneak into the theater.