Unfortunately the American markets are not as densely populated as European or Asian markets.
On top of that we use 3 totally different network technologies that don't always overlap.
Add to that equation the fact that two of the network technologies are proprietary and the usage rights to one of these technologies is controlled by a single handset manufacturer.
The change to an un-subsidized phone for some would require an additional $400 up front... A $200 Early Termination Fee plus a $200 dollar premium for the equipment itself.