said by sbrook:
What it boils down to is that the cost of running more than one cable into an area is too expensive. (Called over-building)
There were a few areas for example, in Colorado, where there were 2 cable operators running side by side, but in the end it just wasn't economically viable.
RCN is an overbuilder in the US. They cover portions of Allentown, Boston, Chicago, Washington D.C., New York City, and Philadelphia. But they don't overlap with 100% of the coverage of each respective cable provider they're competing with in each area or even close to it.