Visual Work, n: Well we sure ain't callin' it a "cash-in."

Advent Children probably succeeded on the corporate tables because FF7 was a huge success that, as DJzen mentioned, left enough loose ends that a plot continuation wouldn't seem out of place. From what i've heard, Advent Children doesn't actually do much for closing the game's story. But because FF7 didn't exactly find closure, people won't inherently reject the thought of a sequel movie on grounds of FF7 itself already having found a sort of completion.

None of the other games, so far as i know anyway, seem to have that. Their stories run their course, find their endings and close things up. Even if someone could find a decent springboard for a tangential sequel, it probably wouldn't find success because it wouldn't "fit" correctly; folks would feel the story was already far too "done."

Again, i don't have much experience in the full FF game spread. But from what i do know, Advent Children seems a special case. i doubt the other games will get story extensions.