system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did internet provider monopolies, such as Comcast, form in the first place? What's keeping the competition out of certain areas so that Comcast is the only option?
gpt
Comcast was a cable television company before it was an ISP.  Decades ago, it was common for a company like Comcast to strike a deal with a town: "give us a monopoly on cable television in this town, and we will wire the *entire* town".

Those exclusive franchises have been outlawed, but the physical networks that were created remain.  It is very difficult to pay for a new network to compete with an established company that installed and payed for its network decades ago.

There *is* competition, for example RCN operates mainly in areas where there was already Comcast service, or some other company.  This is called an *overbuild* network.  It is not, however, a super lucrative business model, which is why you only see it in limited areas.