They can't. I'm 100% sure that Enix signed some deal with Nintendo that involves Dragon Quest and Nintendo. Nintendo publishing the games was probably part of that deal.They can, they just wont because they dont see the series as vital in the west as FF.
I highly doubt Enix signed any deal considering Enix merged with Squaresoft back in 2002/2003 and after that DQ8 was exclusive to PS2 for a long time. I think Squareenix sees Nintendo's portables as a potential sale magnet for the DQ series (but not for the FF series? mind boggling lol).
Anyways, I think after the success of DQXI in the east and the future release of DQXI in the west, I cant see DQ being exclusive to Nintendo anymore.