https://www.cms.gov/CCIIO/Programs-and-I...ty-Map.pdf

What you are seeing is the slow devolution into regional/territorial monopolies. Much like Big Cable is your only option for internet in most places, Obamacare has officially established Big Health Insurance as your only option in most places. While some big cities will have the market share to have competition, most will not as a monopoly will be necessary to make this boondoggle financially worthwhile to the insurers.