A security guy popular in the internet, Ollam, recently left Delta's program because they are changing it to be more pay to play vs miles traveled or something. Delta walked it back but he's sticking to his guns. In his rant, he said something poignant. Something to the effect of "if your significant other raises their hand like they're gonna hit you, but they don't? The time to leave is now." (Video)
Honestly, probably resellers. I use a third party to buy t mobile phone service. Compared to me trying t mobile's home Internet direct, it's a lot better.
But those big three have nothing to do other than cost cut. It's not like they're competing or entering new markets or anything.