I think they probably couldn't port it to 3DS... it's one of the more technically demanding PS2 games coming from Level-5 and we've seen PS2->3DS ports don't go all that smoothly, like with MGS3. Maybe it was a lot of work, they did a lot of work for DQ7 remake and that still sold less than DS remakes, which were a lot less ambitious in scope.
other than 3DS, next viable platform in Japan is PS3, looks like they don't want DQ games on PS3 for whatever reason whether it's a demographic issue or whatever else, not even their expensive MMO which is stuck on soon to be dead consoles right now and they go out of their way to stream it on mobile. The next viable platform is of course mobile in Japan and they're converging all of their efforts there. So it makes sense somewhat.