There are a myriad of factors that I feel are affecting this.

Part of the reason is because FFIII and IV were fun projects from one of the team members who played a large role in their creation, Thus he pretty much was the drive to get the games going. FFV appears not to have anyone from the old days who played a significant role like that, whereas VI does have Kitase but the man seems to be in more of an executive role nowadays. FFI and II by default are so small, they are easy to remake and port. On the other hand, FFV and VI are both rather large games for 16-bit titles, and as FFXIII has kind of shown, I don't feel SE is really well versed in making a next gen game quickly and cheaply without either skipping corners or taking so long it has to be redone for the next console generation that appears.

I feel the size of the games and the lack of a leader to get behind the projects are the major factors that stopped this from happening in the past. This will probably be rectified with the 3DS and PSP2 (I don't remember it's silly acronym) and grabbing a leader should probably be easy cause there are a few people still standing around from the old days. With the Dissidia remakes of Bartz, Exdeath, Gilgamesh, Terra, and Kefka Its not far fetched to see what sticking to the CC/KH engine could do to bring those FF's as remakes.

Still, it would be interesting to see SE try to make them cause it just feels like the teams over there have seriously not tried to make an old school FFs with the old school, some would say, "golden age" RPG design and match it with more modern technology, it would probably be a bigger undertaking than we think despite most of the major design elements are already addressed, but now they have to see if they can actually properly translate their re-imagining of cutscenes and battle systems with some of the flair they've picked up in the last decade and actually get the technology to do what they want or settle for something less.

Though I'm sure the games will probably be redesigned by a new artist, I personally feel SE needs to step up and finally attempt to remake an FF based on Amano's actual designs. Some of his best designs and artwork comes from both of these titles and I would love to see a faithful adaption. That's wishful thinking of course but a man can dream can't he?