Well, there's two aspects at play here.
For starters, you do need to be a certain size as an ISP to really offer cheap services. Imagine, for example, being a town of 500 with a municipal ISP. You'd end up with a lot of equipment to store and maintain and to really be effective, you'd have to employ a bunch of people that spend most of their time doing nothing. (After all, how often would the customer service rep for a small town ACTUALLY be answering any calls?).
That's why a state, county, or national? run ISP would probably make more sense, especially in more rural locations.
That being said, in the UK with BT and the line rentals I described earlier those worked unreasonably well. ISPs could complain to BT to fix problems which ultimately left the various ISPs in a region to compete on tech and price. That was a win for the customer.