The answer is relatively simple. "Memory Restrictions" were the words uttered by Hiromichi Tanaka, producer, at Gamescom last week. According to Vg247, Tanaka said, through a translator:

“The main reason was the memory, On the PC, they have enormous memory. For PS3, there’s a restriction. They are adjusting the memory size and customizing it; it took longer then we were expecting. But we’re really working hard to bring it out as soon as possible."

Tanaka was then asked whether this memory limitation should be attributed to the Blu-ray disc or the PS3's HDD, to which Tanaka responded "memory" and with no help from his translator. So much for nailing down the specifics.

Final Fantasy XIV launches September 30th for the PC. The PS3 version will be available at sometime in March of 2011. An open beta for the game will begin next month.