I think a better way to look at this is to back up a bit.
In the 80's, a cellular phone was a high-end device, and only worked with a single carrier. In the USA, you got it from the carrier and the carrier supported it - you paid what was basically a support contract for it.
I'm not sure what the process was in Europe - I suspect it was spotty.
Europe grew in the 90's into a good modern phone system - partly due to regulations. A phone had to be sold to be work on any network. Another likely driver was the fact that Europe is a more fragmented market: Different carriers have different coverage in different countries, and people (especially businessmen, the first main market) would have to move between them.
The USA didn't have that requirement, so most phones still worked only on the network of the company that was selling them. Therefore the company that was selling them liked to point out their low cost, to get people hooked on the higher monthly contract. In Europe - where a phone worked on any network - this model didn't work, customers could just switch carriers.
The USA is now starting to standardize, but it's taking time. People are just starting to realize they can take a phone to a different network - and most cell companies don't advertize that fact. The bring-your-own device plans are available, but still harder to find. (Though it sounds like they are being advertized more, a good thing.) The bring-your-own carriers have been around a while, but you have to know about them to find them - you can't just walk into a store and find their stuff. You basically have to buy their service from their website.
So the USA is phasing out an old business plan, and there's a lack of awareness of the alternatives. T-Mobile is betting they can grow by advancing the new business model, but is still hampered by it's position in the market: Cell phones require coverage, and they don't have the coverage of the bigger carriers, who are more tied to the older business model. But T-Mobile is making the public aware of the alternatives, which is driving a change in the market as a whole.