Yuji Horii has always said new numerical entries in Dragon Quest would be going to the most popular system whether handheld or a console.
PS2 was the most popular for DQVIII, DS was the most popular for DQIX, and the Wii is the most popular for DQX, it's no surprise. You seem to forget DQI-VI were on Nintendo systems, and DQIV-VI are in the process of getting remade for the DS (VI only one not out in Japan yet)
Yuji Horii is the creator of Dragon Quest, where he puts his games has no relevance on where Kingdom Hearts will go, there's already the KH DS title and there's also a KH PSP title, plus the recent PS2 release KH Re: CoM, a remake of the GBA card-based game.
But once again I wouldn't be surprised if they did release one for the Wii with how many of the consoles have sold.
Another Wii title I'm looking forward to, wonder if Level 5 will also handle development duties for this title.