Before I answer this, let me frame my opinion: I very much want single payer in the United States, and were it plausible to do it on a state-by-state basis, I would support it.
Apparently, states are unable to restrict entry into state benefit programs such as healthcare. They can't set up a waiting period for people moving into the state before they are eligible; that was declared unconstitutional by a previous Supreme Court. So if states were to do their own single payer system, in theory sick people could move into the state and immediately gain free healthcare simply by virtue of being a resident. Conversely, they could then move back to their home state that doesn't charge higher taxes.
So it wouldn't be feasible on a state-by-state basis unless a future SCOTUS reverses their precedent. This is unlikely in the short term, given the current Court's make up.
