On one level, this is because most MMOs are released on PCs. And while several adult and indie games are released on PC each month, Japan doesn't have a sizable PC gaming culture like the West. Most big-named Western PC titles are never even localized—take Star Craft 2 or Diablo III for example.
Another reason for their lack of popularity has nothing to do with hardware, but rather with the cultural psychology of the Japanese people in regards to the internet. In a 2007 survey, when asked "Is the Internet Scary?" 77.3% of the respondents replied in the affirmative. When asked why, they sighted fears of viruses, leaking of their personal data, and being libel for what they said. Even more telling, though, is that these results come from an internet-based survey—a survey from a group of people who are, by definition, at least somewhat knowledgeable of the world wide web.
Despite all this background baggage, Dragon Quest X has the first real chance to bring MMOs to the forefront of Japanese gaming. Not only is it part of a franchise played by many who would never label themselves gamers, but it's also on the Wii—the bestselling home system in Japan of the current generation. And, as I've mentioned before, Dragon Quest X is largely based off of console JRPGs (instead of foreign MMORPGs) and is therefore incredibly user friendly to Japanese MMO virgins. Moreover, with the lack of PvP and general chat, the environment is much more safe and welcoming to new players.
When it all comes down to it, Dragon Quest X is a game designed to ease Japan's rabid Dragon Quest fanbase into the world of MMOs by being accessible, easy to use, and "internet fear" free. If this combination can't make MMOs popular in Japan, I can't imagine what can. And while it "only" sold 420,000 copies opening weekend (which is about a sixth of Dragon Quest IX's opening weekend total) it has already had far and away the most successful MMORPG launch in Japanese history.
Dragon Quest X was released on August 2, 2012, for the Nintendo Wii in Japan. There is currently no word on an international release.