Enix always goes to the console they feel is selling the most. NES & SNES were the dominating consoles of their era so thats why DQ I through VI were on them PS1 & PS2 were dominating in their era so thats why we got DQ VII & VIII. NDS was the top selling console (handheld) so thats why it got DQ IX Wii was the console winner this gen so thats why they put DQ X on it and since Wii U is similar and the newest system, they anticipate similar success for that console so they ported DQ X to it since a port doesn't cost much anyway.
Most people who played DQ as a kid are now businessmen/women who have no tiem to play on the PS3, but who do comute a long way and who own a DS/3DS. Also kids who are getting into DQ would not own a PS3 either, therefore they can make cheaper DQ games for the 3DS and they would still make a lot of money.
PSN: GuiltyPersona Official Foreign JoJo no Kimyou na Bouken #1 Fan Playing: (PS3) Yakuza 5 (PC) Guild Wars 2, War Z (3DS) Layton Vs GS and Animal Crossing.