Cenodoxus posted:

Mainly because cable carriers own the entire local loop and hate competitors. It's also tricky (if not impossible) to compete over the same cable infrastructure because cable is a shared medium - one end of the fiber is connected to the CMTS at the cable company's central office, and the other end of the fiber is in a tiny box on your street where it's converted to coax and shared with the entire block.

Phone lines and DSL are a bit different - your phone line is a single twisted pair that runs all the way back to the central office, and it's yours and yours alone. From there, you've got an incumbent carrier that laid the lines or at least has legal possession of them (the ILEC), and you've got competitive carriers (the CLEC) who are given the right to lease lines from the ILEC at fair rates, and either one can take your twisted pair and give you service over it.

Another factor preventing shared cable markets is that many municipalities offer a pseudo-"protected monopoly" status to cable carriers in exchange for the carrier agreeing to provide service to the entire area. The local government gets their constituents internet access, and the cable company gets so many years of almost guaranteed .