Don't think popularity enters into it. They're clearly doing it because FF10 HD was very successful
Well...I mean they started with FFX, moved on to KH and KH2, then FF9, and now FF12? I think I can see a trajectory here...
Except that KH1 was finished before FF10, and FF9 was likely done by a different team entirely, namely the guys who worked on FF7/8 Steam, which is quite the same (FF10 and 10-2 were both special versions of the game never released stateside, just like FF12 is getting; FF9 is the same exact game with some upscaled polygons, and some little features like Fast Forward mode)
Plus it doesn't account for Type-0 in the mix, which is clearly way less popular than all of those.
I think you're finding a connection that isn't there. Especially since FF10 and FF10-2 are the only games in a comparable position to FF12, namely PS2 games that have versions released only in Japan, and games that have never had legitimate re-releases, making it impossible for anyone to play those games on more modern consoles. FF9 at very least has had a PSN version, for example, so people with PSPs, Vitas, and PS3s could play it, and FF9's remastery isn't anywhere near as intensive as what FF10 got.
Case in point, FF10 got an entirely re-mastered soundtrack, and FF12 is getting the same treatment. FF7-9 notably did not get this.
So I don't think "popularity" had anything to do with it. I'm pretty sure FF12's remastery is a result of FF10(-2) HD's success, since it's the only game that was in a comparable situation, and there is very clearly demand for it, as well the game is very easy to market in the west by saying "Never before released IZJS version!" which, not shockingly, they're actually taking advantage of.