Because they want it on home console, they made it clear, they want it more ambitious , including visually( equal to dragon quest heroes cinematics), and they want to bring dragon quest to the west like they said earlier, so ps4 is the only choice among japanese consoles to do that, and the most important, horii Saïd :
" he said that it is being developed separately, but that Dragon Quest Heroes is being released with the hope that it will build up excitement among those who play it to make them look forward to the next title. "
So after knowing that, its your choice to not see the obvious but your are warned, dont be surprised when its announced for PS4 and maybe Pss3
Dragon Quest games are traditionally exclusive to a.) Nintendo consoles, b.) the most successful systems in Japan, or c.) both. This generation, that's the 3DS. You really think Square Enix cares more about graphical fidelity than pure profit? Tell me why they were so disappointed with Tomb Raider's sales, despite it looking beautiful on every system. If they can get away with making a more inexpensive game and maximizing their profits, they will do it.
DQH being PS3/PS4 exclusive means nothing in this scenario. There hasn't been a mainline Dragon Quest on Playstation platforms in over 10 years, and Squeenix has no reason to mess with success. It would not sell as much on PS4 as it would on 3DS, and that's common knowledge. I'm a realist.