The main reason sales are lower than other Dragon Quest games is because most fans of traditional style RPGs don't like Online games. The reason why Warcraft was successful was because the Warcraft/Starcraft fanbase like online games.
I'm sure that helped a lot with early adoption, but World of Warcraft's success came from far more than just targetting a pre-existing fanbase. It stayed close enough to MMO roots to attract Runescape/Everquest players while shaking things up enough to attract people who would've never considered buying an MMO before - people like me (I've still to this day never played any other MMO's, or any other Warcraft games).
I'm sure DQX isn't helped by being an MMO in a series of traditionally single player JRPG's, and I'm sure it can reach FF XI levels of success easily. But to go any futher than that, I think it needs to be a different type of MMO that it actually is.