My guess is because of the lower price, especially on the CPU side. Both Sony and Microsoft know this is not the golden age of consoles anymore, and no matter what they add in them, people are not going to want to pay a lot of money for them.
I think Nvidia won the design for Valve's own Steambox, though. Not sure what CPU they're using there. Maybe Intel? I have a feeling Valve won't care as much about the price, considering you get much cheaper games on Steam than on PS and Xbox, and he'd rather go for higher performance and launch it at a later date, too.