It’s not just science-fiction filmmakers who are guilty of the “one great scene” thing — look at horror films sometime — but it does crop up in this genre more than most others. Why is that? One reason has to do with the nature of science-fiction filmmaking itself. Science fiction is in general a more spectacularly visual genre than others; right from the beginning, it’s primarily used special and cinematic effects rather than story and character to wow its audience. If you don’t think science-fiction filmmakers know this and capitalize on it, consider the fact that in each of the scenes I mention above the actors are silent or very nearly so. By stripping away the things these films and filmmakers generally do poorly or at least less well — character, plot, and dialogue — they can focus on what they are excellent at and dazzle us thereby.
But another reason is that science-fiction audiences let filmmakers get away with it. The people I know who came out of The Phantom Menace tended to say that the film had only two scenes worth watching — the aforementioned and the pod race (another low-dialogue visual scene) — but that those scenes were really cool. As bad as Phantom was, these two scenes were sufficient in some ways to excuse the rest of the film. Lucas would use this “Well, the movie was bad, but that scene was cool” rationalization that science-fiction audiences employ in the other two films of the prequel trilogy as well, banking on Yoda’s spinning-lightsaber acrobatics in Attack of the Clones and Obi-Wan and Anakin’s climactic duel in Revenge of the Sith to give folks their required jolt. It’s a crutch, in other words. But inasmuch as audiences give science-fiction filmmakers that crutch to use, I don’t know how much you can blame the filmmakers when they use it.
Awesome scenes are awesome, but I go to the movies to see movies, not just scenes.