A big problem is that RPG gameplay is kind of, well, bad. There's a reason that Final Fantasy does well, and it's because their developers tend to try to invent a system that is replayable and customizable, but at the same time not bogged down in administrative details. In general the FF series is one of the few Japanese-style RPGs that try to keep themselves up to date and that's why they keep doing well.
A lot of other RPGs unfortunately are, well, boring. They have a few quirky features or some hook, but are otherwise the same game as every other game in the genre. So, once you're done with the story, you don't want to play anymore because the game itself is bad.