The developers have already stated why. It was too hard to do towns and minigames in HD. That is a factual statement they made.

Final Fantasy XIII producers: Traditional RPG towns are tough to do in HD

In a recent interview with Ultimania, they say that the limit isn't the HD technology, it's the amount of artisanship. "Considering the amount of work to make graphics that deserve HD, it is hard to make towns in the conventional style," said Toriyama. FFVII's graphics were beautiful at the time, but when you consider that we all played them on a 4:3 TV SD screen rather than the 1080p HD behemoths we're all playing games on today, you realize that it's a lot more work for artists to put in to make the game look as epic as Final Fantasy should. That's one reason why FFXIII doesn't have conventional RPG towns.
Kitase agrees, saying that "it is very hard to make games on PlayStation 3 in the same style the games in that era had. Making graphics will take enormous time." So score one for standard definition and the limits of the old pre-HD consoles. Note that neither producer says it's impossible, just a lot of work. So maybe some enterprising young developer will come along and make a solid, HD JRPG in the old-school fashion.
Please will we get away from defending this game by refusing to accept what the developers are telling us are the reasons. Graphics are now coming first to gameplay.

On NPC, saying that the plot is the reason is a MONUMENTAL cop out.