The game is so old that the graphics would need to be all done again, there would be just too much work for a remake that just won't sell as well as the next new Final Fantasy game. A game like X is much easier to give a HD re release because everything is 3d models all ready so they just need to res up all the textures.
The next new FF game is XIII-3 and considering that the sales dropped a few million between FFXIII and XIII-2 I doubt that the next game will do that well.
On the other hand, VII is often considered the best in the series (not by me) and has a legion of fans that have been asking for a remake for years. I am sure a VII remake would sell far better than XIII-3 could ever hope to do.