The reason to set something in the future is almost always to say something about our current world state and heighten it to present a point. Drama is conflict, so this is usually done by presenting something that's bad now/has the capacity to turn bad, and then dial it up to 11 in the future. So yeah, if the future is idyllic, what's the point in it being set there? I'm not saying there'd be no reason to do so, but by and large, that isn't why we look to science fiction.
Now, it also depends on what your definition of utopian/dystopian is. Because there are quite a few films set in worlds that are 'utopian' in a traditional Western sense that are deliberately using that backdrop as irony to present something darker underneath. So for example, yeah, you're right, Minority Report is ultimately a pretty dystopian film when we explore its themes and moral quandaries, but for most people living in that space, its world is pretty utopian. Crime is at an all time low, the economy seems to be doing pretty well, the cities are thriving, it seems all-round pretty sustainable.
It's a hell of a lot more utopian-looking than say Children of Men or Blade Runner where society is utterly collapsing in on itself. It's only that it's got this extremely problematic technology at the centre of the story that makes the world feel so dystopian (And I guess the surveillance/privacy/data tracking stuff). Same could be said of Gattaca. And Her. Even the OP example of BTTF2, as AniHawk mentions above, is poking fun at the corporate/commercialised world of the 80s, so does that make it a dystopia?
So there is often a really nice spread of environments these films are set in, even if they can largely be defined as 'dystopian' in short.
