I'm guessing this means that subscription rates aren't that high, rather than the sales of the actual game itself aren't that high. We've got DQX at 630k sold, which is a respectable number for Japan alone imo.
Games like this get their money back through continued subscriptions more than anything else. I'm sure - if the game hasn't already returned its development costs - then it will do swiftly, and continue to make money on top of that. But I'd guess Square-Enix had unrealistic expectations and likely expected this to be a cash cow they could milk for years to come. Which they probably still can, but not with quite the success they'd have been initially hoping for.
Seems unlikely that the Wii-U version will give it too much of a boost either. I'm sure some people will be holding out for that version. But I bet Squeenix weren't hoping to be pegged as the only big Q1 release in Japan, so it again - probably won't provide the boost they were hoping.
Missed opportunity. Just developing another Dragon Quest mainline game would surely have been better, while they continued to work out how & when to deliver a Dragon Quest MMO.