Their numbered games take too much time, they're already working on FF13 way longer than other FFs and have announced that it will have to sell 10x as much to be successful. IMO it's shaping up to be another TSW and they probably won't be able to keep up their 3 numbered FFs per gen rate this time.
Sequels to specific games allows them to reuse the characters and have people connect with them better, also filtering for those that had a good fan reaction (note how there are no sequels to, say, FF8).