The one excuse (which is valid) is that that the 'Cubes user base is small, add the fact that the 3rd party releases sell the least, why go and take needless losses? I'm not going to play the blame game or yak about how Nintendo needs to get the online facet in gear, that argument has been brought up time and time again.
Looks like the 2 superpowers have reached an agreement. Very scary. Put in XSN, and one could say that MS took the cake. Imagine, Madden+XSN= Killer App.