In The Beginning, Ma Bell was a "natural monopoly." Imagine if competing phone companies were running around, each putting up posts and running wires, each with its own subscriber base that could talk amongst themselves, but not to other companies' subscribers without paying stiff fees (if at all). It would have been hard to get the thing off the ground.
When the technology made it feasible for other companies to handle the switching and transmission from the customer's one wire pair, first in long distance and then in local service, the monopoly no longer made sense.
Actually, let me rephrse -- much smaller, local monopolies made sense. I can choose from a number of cable and DSL providers, but Bell South still maintains the one and only copper pair coming to my house. I can choose from among seferal natural gas providers, but Atlanta Gas Light still runs the one gas line coming to my house. The monopolies became smaller because it became more feasible for a small last-mile monopoly to provide access to multiple providers.
Back in the day of telegraph there were multiple companies setting up point to point wires for businesses. So you’d have a private line between the factory on the outskirts of town and the office in the center of the city. Even with the very basic technology, it was a mess.