Because Sony/MS want to control the game
As stated in an iPlayer blog yesterday, Sony and MS don't want some upstart creating a nice online television service without input from themselves.
Probably because it won't generate them any revenue (in the case of money-grabbing Microsoft), or that i'll make their own efforts look ridiculous (in the case of forever-behind-the-curve Sony).
http://www.bbc.co.uk/blogs/technology/2008/04/wii.html