Just, considering the democratic ideal of the government serving the people, I struggle to come up with a plausible justification that the states might give for denying municipalities and I'm hoping someone more familiar with the matter here at HN can shed some light.
I noticed that line of argument seemed more common when I lived in the south (NC).
In terms of politics, cable providers have lobbied to carve out a legal monopoly for themselves, and so, in effect we see the same effect from the private company.
The answer, in my opinion, is to have the municipalities run empty conduit to every home, and allow any private company to lay their cable there. This would allow more competition for electricity, cable, and potentially other utilities.
[0]: https://en.wikipedia.org/wiki/Blue_laws_in_the_United_States
I can come up with a plausible explanation for that since it does seem to serve the will of a lot of people who also happen to be people that vote heavily -- especially considering the time period the law was made.
For banning municipal broadband, though? I can't think of any way that serves any significant bloc of people in the state.