They could, and in fact the medicare block grants in Obamacare are for that purpose. States accepting the block grants expanded access to medicare, massively reducing the number of uninsured. It doesn't have to be an every-state solution, but like any insurance it benefits from economies of scale.
Why don't they? There's a lot of people in CA and NY who don't think it's more affordable and better, notwithstanding 20 or 30 countries who have done it successfully. The private insurance industry is, understandably, a huge lobby against it since it would likely reduce or eliminate their business (the mandate to buy insurance, under Obamacare, was pretty explicitly a payoff to the insurers to accept it because they would receive a huge influx of new customers), etc.
Why don't they? There's a lot of people in CA and NY who don't think it's more affordable and better, notwithstanding 20 or 30 countries who have done it successfully. The private insurance industry is, understandably, a huge lobby against it since it would likely reduce or eliminate their business (the mandate to buy insurance, under Obamacare, was pretty explicitly a payoff to the insurers to accept it because they would receive a huge influx of new customers), etc.