I wonder how much has to do with the cost-optimization pressure on a private carrier. Building robust infrastructure is only valuable to them insofar as it enables them to continue to earn money. Reliable service for the sake of reliable service--especially to residential regions--is a non-starter.

Less pessimistically, it's possible that the regions that're affected have natural "choke points" for infrastructure, either moving through the mountains or along highways, etc., which of course leads to a single-point-of-failure, though it may be unlikely.