There are a lot of things worth leaving to the states, but the way insurance works, the financial feasibility depends a lot on how big the buying group is. It's why healthcare bought on behalf of a company is cheaper and better than healthcare bought by an individual, and it's why PPACA created marketplaces for individual insurance buyers.

It's almost certain there are things that are feasible when the US has a certain healthcare policy that aren't feasible on the state level. I'd guess that single payer is one of them.