Anyone can start an ISP as long as they are willing to pay for the infrastructure to deliver the last mile connection to their customers.
I've always said, the battle for broadband isn't at the national, state, or even regional utility level. It's in the city utility easements.
City and county governments make exclusivity deals with providers. Back when DSL was first rolling out in the late 90s, states mandated that the monopoly easement holders offer their copper wire and telecom junction box space to competitors in return for their cabling monopoly. The phone companies tore the startups to pieces with bogus charges and quality problems, insuring that the phone company service worked OK while the competitors' equipment worked like crap and would never make any money. On the argument that they could provide better service, the phone companies lobbied to get the competition requirements pulled, and they have for the most part.
The cable and phone companies will never, ever allow a competing wired standard into the utility easements. They will fight it at every level, and throw obscene amounts of money around. Only a handful of super-rich companies have managed to bust these agreements. Google Fiber, for example, in very limited areas.
And if you wonder whether they have been successful, check your junction box and see how many data-capable cables are currently entering it. I'm betting it's 1 or 2, and those probably belong to the phone and cable company that have been operating in your locality for at least 30 years.