Old horse, that's been beaten more than enough here (was another a thread about it on the front page until last week or so). SE made up their minds a long time ago on this issue (like, 4 or 5 years ago, if not further back). Link to transcription of an E3-2009 interview found via a quick google search:

http://www.fftogether.com/forum/index.php?topic=1631.0
Q: Why did you decide to make a new game rather than continue to evolve FFXI?
A: It all comes down to, originally, FFXI was designed for the PS2. It then moved to PC and Xbox 360. A lot of people have asked for a port to the PS3. With an MMORPG of this size, you have to develop new content. Porting to a new technology would take a lot of effort. Instead, we decided to use that time to make something new.