What salient factors distinguish a cable company from a "mere" streaming video service? Demand programming? Nope. Time shifting? Nope. Advertisement? Nope. Other than the literal media the data flows over, what is the legally meaningful difference?
Cable isn't a natural monopoly, they are sanctioned monopolies due to their negotiations with municipalities; Previously their only competition was over-the-air broadcast. We have this model in place of a company being given sole propriety over the physical 'cable' infrastructure.
But, just look at what has happened now that Verizon can compete by using a different set of wires (FIOS). They want to compete because it is a huge business and the monopoly has been broken.
The difference with a video streaming service is that they don't own the infrastructure over which their service is delivered.