Maryland has had an all-payer system for a long time now
Sure, but the state already made adjustments to their model a few years ago to limit hospital costs. Another major change within the next decade to the healthcare system in the state could prove to be quite the destabilizing force for those in the health industry, not to mention the political backlash that would emerge from the kind of tax that would need to be levied to pay for single-payer.
Again, another reason why I don't see Jealous getting the nomination, or being a strong general election candidate. Maryland traditionally supports establishment Democrats, and when an outsider get the nomination, it votes Republican (see Maryland's 1966 Gubernatorial election).
I can see a candidate supporting a soft version of single-payer, but the health industry being a dominant employer, it'll take a few years for the state to full-out accept it.