A cable company is a natural monopoly; servicing an area requires a large infrastructure investment, there is usually only one cable company willing to service a given household, and cable companies negotiate with municipalities. A video streaming service doesn't have these properties. This is a legally meaningful difference because much of the regulation imposed on cable companies is intended to compensate for lack of competition.
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.
