Monopolies can raise prices, decrease product output and quality, and capture huge profits.
One trouble is that excites those "greedy capitalists" who see this as an opportunity to capture some of those profits for themselves, taking marketshare away from an abusive and disreputable service provider, by offering a better deal to willing customers. They can even pre-sign customers like Google Fiber did. They could even issue bonds or start crowdfunding campaigns to bootstrap the investment.
Comcast can't do zlich to prevent such competition. The municipalities on the other hand have the real monopoly as they control rights of way (utility poles, trenches, roads). Worse, they often grant exclusivity deals to motivate the few providers that are considering jumping through the piles of red tape. Municipalities do this with the motivation to get more taxes, but this really screws up their constituencies.