States commonly have more internally aligned political goals and therefore can move quicker. It also has the benefit of allowing different states to experiment with different solutions and all states can learn from the results.
Lastly, inter-state mobility is much higher than inter-country and this will allow residents to congregate in a state that matches their political desires. E.g., if someone doesn't like the tax burden of states that provide more services then they can move to a state that provides fewer.
This isn't really feasible because residents of one state with no universal healthcare + lower taxes could easily move to another state with universal healthcare + higher taxes when they get a serious illness.
https://www.latimes.com/archives/la-xpm-1999-may-18-mn-38325...
[0] https://repository.law.umich.edu/mlr/vol104/iss3/5/ [1] https://www.law.cornell.edu/constitution-conan/amendment-14/...
"If the purpose of the requirements was to inhibit migration by needy persons into the state or to bar the entry of those who came from low-paying states to higher-paying ones in order to collect greater benefits, the Court said, the purpose was impermissible"
It's obviously hard to know for certain what SCOTUS will do, but under that standard it sounds like having health care benefits only available to long term residents would be on very shaky legal ground.
Determining residency is not rocket surgery; could steal the entire system from state income tax codes.
If a state provided universal healthcare for residents, it'd quickly be overwhelmed with "hard cases" from other states where coverage is a problem. Which would drive up costs, and taxes, and drive productive/healthy people out.
See the current inter-state shipping of homeless people, especially those who suffer mental illness, that is often done via bus tickets paid for by local governments looking to shed those people.
In general, we don't dump problems on to other provinces. It's bad form and federal courts will fix that quick especially with so much money on the line. If Florida state sends patients to a universal health care NY state for care, you can bet, NY State will be suing either residents of Florida for medical bills or Florida State directly.
All of Canada has universal healthcare, which again makes Canada a completely different discussion from my earlier comment. There isn't the kind of huge discrepancy between provinces as we'd have if some US states had universal healthcare and some had the status quo.
To your argument about states suing one another - we literally already have localities giving mentally ill people bus tickets out of state, and that's been unable to be stopped via the court system. I'm not dealing in hypotheticals here, I'm talking about things that already happen.
That was kind of the point OP was trying to make - in the US there are quite a few states that would do this without blushing. Without the Federal government instituting a framework and legal mechanism to prevent it, there is no way any individual state can do it on their own.
Any state trying to implement a single payer system would get crushed as tons of individuals and even other states tried to milk the system (like putting homeless people on a bus with a one way ticket).
States would need a "suicide pact" of sorts to ensure a bunch of states made the leap at the same time. There actually is such a plan to subvert the electoral college to ensure the popular vote winner always wins the presidency, but that's a relatively simple pact around single law. Health care laws a incredibly complex and rather than try some kind of weird multi-state pact you're better off fighting for a national law.