Except the Gamecube had Japanese support. Xbox doesn't sell hardly at all in an entire region that Nintendo does well in.
This time around, you're comparing something that already has more than 50% marketshare to a machine that has no presence in Japan and minimal presence in PAL territories.
This is only NPD, USA. Japan support? We talk about US... The GC has won thanks a massive pricedrop and Mario Kart.