Only problem is many people, like me, cannot actually switch. Comcast is the only provider in my apartment. No DSL company would hook me up and no other cable company has wires in here.
Consider the situation in which most subscribers of Comcast in your area wanted to switch..if there are enough of them to build a business, that strong 'latent' demand is a market waiting to happen.
It doesn't work like that, not even in theory. Communications infrastructure is a market that inevitably involves natural monopolies: first because of the cost structure of the industry (marginal cost curve always decreasing) and second because of network effects.