If you haven't heard of Dragon Quest, it's a classic JRPG that used to be as big as Final Fantasy, at least in Japan.
Dragon Quest X, the only entry in the series not to be released outside of Japan, is an MMO that combines the traditional Dragon Quest RPG elements with modern MMO mechanics.
A multiplayer Dragon Quest isn't something we usually get, so missing out on this one was a big disappointment.
Square Enix has made many comments about why we haven't seen a North American release. They've talked about how they don't have the infrastructure to host servers in NA and how the game series isn't as popular as it once was.
What it really comes down to is that entering the North American MMORPG market that's already dominated by World of Warcraft might be hard for a game like Dragon Quest.