Nothing is really stopping new entrants from joining the market -- other than the fact that deploying a broadband network in the US is really, really expensive. In Europe or Asia you can much more easily obtain a license to run cable across a wide area encompassing a whole city; in the US those rights are split between dozens of different municipalities, some of which don't have the resources to approve requests.
Additionally, US telecom regulation requires that if you're going to cover a city, you have to cover the WHOLE city; or at least you can't discriminate based on income. This is why Verizon has slowed FiOS deployments to new developments only -- it's a way to be able to only roll FiOS out to wealthy customers. People in the ghetto don't buy broadband, but if you want to roll out in a US city today, you have to make it available to them anyway.