The games were completely redone in order to milk them for more sales.
Well not exactly, but there's no secret magic way to port games -- you have to rewrite all platform-dependant code or, which is more common nowadays, work on different platforms in parallel.
Actually, the ports were done to update the codebase to something more portable. Square'a shareholders were really nagging on the board of directors to diversify ther products
The FF7 PC port was an attempt to diversify, remember that FF1 and FF2 were wonderswan games first.
FF1-6 were written in a very platfrom dependant version of assembly code. Square decided to update the games so that if a new system came out, they had the ability to port them without having to rewrite the games in the future.
So far it's worked, now they can make FF1-6 for the PSP and not have to worry about rewriting the games again.
