Or, nefarious "JCs" who don't want it to work? :) I doubt that a state's attempt happened in a vacuum without "input" from interested parties elsewhere in the country that did not want the status quo upset.

Perhaps, but that doesn't seem to be the main reason. The then-governor was hugely invested in it, and had gotten strong support from the legislature, and it was immensely popular. But they couldn't make the money work:

The financing model Shumlin rejected would have paid for a top-shelf benefits package, one on par with the plan enjoyed by Vermont state employees. Shumlin said the state could have reduced pressure on the financing plan by reducing the quality of the benefits package. But he said Vermonters by and large have high-quality plans in the current system.

Offering a lower-value plan, he said, would have resulted in too many residents paying more money for worse coverage in a publicly financed system.

Shumlin says he doesn’t regret promising his supporters a single-payer system that he’s now learned he won’t be able to deliver.

vermontpublic.org - Shumlin its not the right time for single payer

This is the main problem for single-payer health care financing. The money. It's very expensive to give everyone top-flight insurance. But in the current system, a large number of folks (a minority but a sizable minority) have very good health care plans for which they don't pay very high premiums. The "Cadillac" plans, mostly held by unionized workers both private and public. If you give everyone a Cadillac plan - or even a plan as good as a standard large group health plan - the system can't work without very large tax increases. If you don't give everyone a Cadillac plan, then a large number of people end up with worse coverage (and probably paying more money for it). A large enough number to be fatal.

Put another way, it's a variant of the Wobegone effect. Half the voters in your state have insurance that's better than average. If you set up a single-payer system that provides better than average coverage, you can't afford it. But if it doesn't provide better than average coverage, then half your voters will see their quality of coverage get worse under your new single-payer system. And in all likelihood, it's not "average" that's the political killer - whoever's got the top 20% quality health coverage in the state is probably where you need to be, because if you make 20% of your voters' health insurance worth, it will kill the proposal.

It's not the JC's. Or rather, it's not just the JC's. You can't make the switch without making a large number of ordinary voters worse off, because the money just can't work out. It's easy to blame the JC's, because of course there are some JC's who are against single-payer health care. But even in states where the political decision is made to blow off those JC's, you still can't square the circle and give people insurance that's above current average and find a way to pay for it.