That is only because CDMA had such a jump on GSM in the USA, and the GSM carriers in the USA are still smaller. Verizon also has a massive analog network that T-Mobile won't touch as their phones are GSM only.
Unfortunately, the free market approach adopted when installing networks in the USA has led to a number of problems, and while you might think Verizon service is good relative to the other US carriers, it cannot compare to the carriers in Europe who use a shared GSM network that was adopted after much deliberation. Like most of the world, we are GSM-only.