Why Can't the States Do This?

If the individual states have the authority to regulate which healthcare insurance can be purchased in their own state and this unavailability of choice for their citizens is a major cause of high insurance costs (and I believe it is), why, pray tell, doesn’t each state allow all insurance companies to compete within their borders, for the benefit of their own people? Would not this simple tactic be advantageous to all and, at the same time, help to forestall the looming government plan by addressing the topic of affordability? Or, am I all wet?