Sales in Japan are just a single factor.
There are many others.
*Deals between companies (you know Resident Evil going to Gamecube despite the PS2 success, Monster Hunter going to 3DS at a time where nothing could be said about Vita's success and all the fanbase was on PSP, Tomb Raider going first to Xbox despite the fanbase being all on PS/PC).
*Strategic goals for the series thinking outside of Japan.
*Strategic goals of the whole company trying to be successful at cultivating an ecosystem where you can play all their major games on a single very successful console (like in the PS1 and PS2 days).
*A creator thinking that after so many years it's time to move on from being stucked in the PS2 generation because of certain hardware limitations hence the desire to try to go big and ambitious again like they were with DQ8 in 2004?
In any case fact is that the creator has already said to expect the game on home consoles, so certain options are already out of the picture unless we're willing to say that he might be lying
Some people want to argue that for them this is not the right decision? We can discuss about that but it's kinda pointless without knowing the details involved, also the game is still NOT announced.
Some people want to stick to a single argument, the japanese sales, to say that despite what Horii is saying, the game will still be definetly for 3DS? That's delusional.
Btw this is premature, the game has to be announced first, hopefully at TGS?