Because the next Playstation Generation will probably support DirectX 11. And if FFXIV ever gets both feet out of the grave, it will last long enough to see a re-release in the next console generation. So supporting DX11 now would be a good money-saving move in the long term.
You are asking the money guys to allow the programmers to waste time on something that might happen after they win the herculean war or making the game a success on PS3? (pc version of this game is a non argument, its dead). Gl with that. I'm amazed someone talked them into carrying on with the game to try bank on PS3 being a success as it is let alone worrying about that far ahead in the future, PC version will get the platform specific upgrades its playerbase size deserves (none).

Any other MMO that has/will do this kind of stuff is either PC only or has a major pc presence, FFXIV PC is so small if it tried to ever run as just a PC game it would be dead before they finished the sentence to suggest it. FFXIV will never have to worry about saying PS3 limitations because it will live or die on the PS3, PS3 is your hand of god so better get used to it and not worry about it limiting what would be a corpse of a game.