Because cable companies (which became cable ISPs) weren't originally something you could call a public utility. When they started, nobody knew what was the best way to rig up houses, or allocate bandwidth. When they started offering Internet service, that increased the complexity because now each home needed to be able to transmit data back to the cable company. These were all complex problems with a plethora of possible solutions. The "myth of capitalist efficiency" is precisely what filtered out the bad solutions over three decades, leaving only the efficient ones.
Who cares about the cable companies? If we had a public data utility, we'd have just strung fiber everywhere and no one would have had to solve the problem of transmitting upstream over coax. Then those who wanted to continue to be robbed by the cable companies could continue to do so while the rest of us purchased our data services a la carte. Heck, the cable companies could even provide their services over the new infrastructure.
This is what we were supposed to get via our existing common carrier infrastructure if the phone companies hadn't stolen all the money for it. Since private industry has proven itself too inept or crooked to get this done, I think it's high time we take it over as a public works project.