Lack of competition is more about the cost to establish service in an area, and the ROI on service in an area with competition. It costs a lot to pull wires past a lot of potential customers and if many of them won't sign up because they already have a good enough option, it doesn't make much sense to do it.
Cable and Telco compete because when cable was built, it was a completely different service, but they've both evolved to fill the same role.
This is why mandatory line sharing is important for competition, and it's in the telecom act of 1996. But the FCC first said it only applied to telecoms, and then said it doesn't apply in remote terminals because of lack of space, and then courts said it doesn't apply at all because telecom and not cable isn't fair.