It's not cut and dried.
Todd below described what networks are, but the nut of it is that before you go into a state to sell your health insurance, you better be sure you can get enough business in that state to have the numbers of insured high enough that you can negotiate reasonable (read: profitable) fees with that state's healthcare providers. Simply put, you've got to be sure you can make money in the state first.
You can't look at our system with the belief that our health insurance industry is designed to provide health care for our population. It isn't. Our health insurance industry is designed (like everything else) to bring value to the shareholders. Having to pay for somebody's actual health care is just "a cost of doing business" that is avoided wherever possible. Our health insurance industry is in the business of collecting premiums, not paying claims.
You can't look at our system with the belief that our health insurance industry is designed to provide health care for our population. It isn't. Our health insurance industry is designed (like everything else) to bring value to the shareholders. Having to pay for somebody's actual health care is just "a cost of doing business" that is avoided wherever possible. Our health insurance industry is in the business of collecting premiums, not paying claims.