Because despite the limitations, FFXIV is being considered a success, so on one hand there is no need to make a new MMO, and on the other hand, if they did make a new MMO, it would just be a FFXIV-2 on a lot of things (no customisation, trash quests in a useless open world, linear instanced donjons, fan-service, overdrawn story with botched cut-scenes, etc.)

Another thing is FFXIV is kind of a thorn in SE foot. They certainly don't want to make a MMO now, and they probably didn't want to make one around 2012 if they didn't have FFXIV to rebuild.