Well it is a bit shabby considering how big the title is in Japan and that the ps2 has a 20million uberbase over there. For example, pokemon on the DS sold 5million times already I believe with a userbase not even half as big as the ps2 has in Japan.
DQ from what I recall though didn't have nearly the amount of production value/polish that a typical FF game gets these days though. Could be the populous rejected* their effort.
*if you can call 4 million or 25% of the userbase a rejection