Everyone who wants DQ will buy it whatever system it's on, so financially it's not in SE's interests to spend money changing platform. It might be that it still gets released on Wii but around the time of Cafe's release - that way non-Wii owning DQ fans could buy a Cafe and play it on that instead (Cafe being backwards compatible). Some reports suggest that a similar effect took place with people buying a 3DS and a copy of DS Pokemon Black/White to go with it.