Handhelds are different.
Plus, Nintendo DS has had 2 remodels which have done wonders for it's sales (people love handheld remodels) and let's be honest, the PSP is pretty terrible with keeping up with DS.
While DS gets the majority of software and 3rd party love, PS3 and 360 still have amazing support (more than they should imo). This is due to the reluctance of developers.
Simply put, Wii has more competition than the DS does, it is not in it's own market like many would think. DS also has the success and software support of previous generations to keep it going, Wii still has to fight for every last scrap.







