Because the 3G infrastructure is overloaded as it is without multi-gig downloads?
Carriers here like to complain that people are downloading too much rather than just upgrading the infrastructure to deal with it. I shudder to think what would happen if people started downloading over the air on a serious scale, it's already pretty bad with normal web surfing.
I had some thoughts on it recently: http://www.reynoldsftw.com/2010/04/imagine-a-wireless-sync-w...