Porting up from the technicaly weakest console made sense when power difference between the consoles were small.
The XBOX 360 & PS3 are at the very least 2-3X more powerful than the Wii ( this is just a guess , in reality they are much more powerful) Porting games from the Wii would bring about a standard the the HD gamer wouldn't respond well to.
I think developers who ported from the Wii would loose a competetive edge against those who developed for HD , it's not financialy viable IMO .