Hmmm... China has both GSM and CDMA networks, and has about 650 MILLION users, and geographically is about the same size as the US, so it’s not the network diversity or the size of the market.
I think it’s just the fact the carriers don’t want to share “their network” for free tasks like “pay by phone”. They’d want to charge you for it, somehow. Yet the first one who offered it, and hooked up with Safeway/McDonalds/Starbucks would make a KILLING by gaining new subscribers.
only a marxist would think “pay by phone” wasn’t orwellian.