Well, I always saw it this way: they rarely visited Earth on the show, so the films were their chance to show it. As for why they kept coming back, I don't know. I guess they needed a grander scope. Generations and Insurrection both feel like the smallest of the ST films. Something about showing Earth and then showing deeper parts of the galaxy subtly expands the scope of the film, for whatever reason. Or maybe it doesn't. That's just how I feel.

Anyway, it made sense in the last one (origin story), and my guess is the reason for this one is that the bad guy (whoever it ends up being) attacks Earth, which would make threat more personal, I guess.

I definitely agree about The Voyage Home, though. Love the movie, but it's all on Earth! In the 80s!