Probably because it's easier to prevent the companies from abusing their position than it is to do something about the fact that the monopolies exist in the first place.

The former is a matter of defining a rule that says "If you do this, we will fine you or worse", where "this" is defined as various anti-competitive, anti-consumer practices.

The latter is a morass of legal wrangling (think ma bell getting broken up) that would have to repeat itself for every single major ISP out there.