Well I think the "Final Fantasy" name lost a lot of weight with XIII to be honest, despite it's sales (before people knew for sure whatit was a like) I think a lot of people who might have been Final Fantasy MMORPGers became a lot pickier, and also decided to take a wait and see attitude when it came to the new outlook of Square-Enix.
On top of that, above and beyond the bugs I think there is an issue with other MMORPG developments. I mean we have "Cataclysm" coming out right around the corner, and while DC Universe Online has apparently been pushed up by many annoucements (six months in fact) that news hasn't seemed to sink in everywhere and a lot of people are still eagerly awaiting that one like it's coming out in a couple of weeks. Then you have to consider that early next year we're going to be seeing "Old Republic Online" which is probably one of the most hotly anticipated MMOs of all time, and the one a lot of people think might actually steal some of Blizzard's thunder (though let's be honest, even if it does, Blizzard is still going to be filling up another Scrooge Mcduck type Money Bin from Cataclysm).
I think the game kind of hit a perfect storm of a bad release time due to other games, a flagging brand name, and releasing a game in a typically crappy condition to be "fixed later, once we are making money" which works for other MMOs, especially highly antiipated ones, but given the other factors this game didn't have the anticipation it's brand name would normally garner due to FF XIII, and a ton of quality competition. I mean even if the game itself seems to be something of a dark horse contender so far, DC Universe Online, has the name and backing of one of the biggest comics companies/franchises of all time. On the best of days I don't think "Final Fantasy" ever had that much oomph, even when it hadn't been so badly compromised due to a disappointing release and even more disappointing attitudes on the part of it's creators.