Maeg: Mostly, what kara said. It doesn't need to break up the plot or action at all, and in fact these things are usually better the more you understand what's going on, never mind that it's more immersive and interesting as a story if it appears to all be in the realm of the possible, that it might happen sometime in the future, rather than just being something somebody thought up the night after too much bad pizza. The problem is, it involves actually understanding several fields of science, and, while there are few limitations if you care to think of a way something could happen, those limits are still there.
Besides, I can name dozens of hard SF movies. 2001 being the most famous (and I don't ask for it to the extent 2001 had, where there was no sound in space and ****), of course, but far from the only.
Really, the reason you never see any these days is because education is such ****e that you'd get people saying "Hold on, why isn't stuff slowing down when it doesn't have an engine actively burning behind it for a minute?"
You'll also notice that Star Wars is a fair sight more accurate than most sci-fi on TV. It was fluff in many respects, but occasionally they took efforts to get things right. They NEVER do anymore.
Beh. Anyway, if anyone is stupid enough to ask why in MY renders and movies things can be moving without thrusters being on all the time, or starts going on about how the heatsink for a fusion-fission powerplant wouldn't need to be that big, I'm going to do my best to burn their ears (eyes?) off. You've all been warned.