Didn't stop them from including basketball. Took years for the internation teams to put up a fight. The air-time issues mentioned by Icy is a much greater reason as to why it would make little sense.
The difference with football is America would literally kill every team. Basketball it's fine losing by 60. It's still fun to watch a blowout. They would be carting players off left and right if they added football.
I understand rugby is rough. But the players playing football are just too big and strong. You have to be big and physical up front. Nobody would match up at all with USA. I mean we're talking 6'7" 300 lbs.