Unlike almost every other country in the world, the US has the absurd situation of mobile-phones being locked to specific carriers. Even if you get an unlocked phone, you're still limited to moving between carriers that kind of radio network that your phone supports. This leads to fragmentation of the market.
A consumer should be able to buy any phone they want from any vendor they want, and use it on any mobile carrier's network. Only when that happens will there be true competition.
Imagine if each car maker required you to use their own brand of gas? Or only be able to drive on highways that were part of their network? Or if every computer manufacturer ran their own data network?