Exclusive municipal franchise agreements have been prohibited since the Cable Television Consumer Protection and Competition Act of 1992.

Your city cannot unreasonably refuse to grant a franchise to another operator, in the unlikely event another operator would like to provide a similar service. These things used to be a government-granted monopoly, but IMHO, the continuing lack of competition stems from market realities.

Building a new network in a city that's already covered adequately is hard to justify economically, because switching is a pain, and most of the market can't really tell the difference between providers; and even if you do start to attract customers, the competitor's network likely gets better as you relieve their bottlenecks.

Targeting only the areas of the city that aren't well served is likely to not be economically viable either. For one thing, the franchise agreement might have a minimum coverage percent. For another, there are probably reasons the other network doesn't cover those areas; some of which may still apply.

And there's the elephant in the room, the incumbent presumably has enough revenue from other areas that they can cross-subsidize network updates and build out to compete where you are. In some ways, this was Google Fiber's market strategy: Google Fiber's corporate goal was to get more people connected at high speeds so that they could load higher quality ads on YouTube and other Google properties. Building out fiber in the Kansas Cities was a way to meet that goal in those cities, but it was a lot of work. By announcing planned cities in early 2014 [1], Google was able to induce incumbents to improve their networks in those cities, reaching their goal before any Google Fiber permits were approved in those cities; then they could wrap it up and end all expansion works. :P

[1] https://blog.google/alphabet/exploring-new-cities-for-google...