One other reason is that counties have the land rights. They negotiate with the big Telco's like Comcast, Verizon, Cox, Charter, etc and then issue a monopoly or duopoly in the county for a fixed period of time - usually 10 years at once.

The Telco can offer a few things like fiber connectivity between county buildings, tax payments, etc. By doing that, they secure the rights to the area. Given the federal system of the United States, this is a difficult thing to stop.

This makes true market-based competition within most areas very difficult to achieve.