We live in a country in which Democratic Congressional majorities could not pass a public option, and in which Republican Congressional majorities have voted to repeal or cripple the Affordable Care Act well over fifty times. Single-payer healthcare could not be implemented in Bernie Sanders' own state of Vermont in 2014, where popular and institutional support is incomparably more congenial to such a system than in most other states.
Yes, there is a difference between explaining how a single-payer system provides equitable, effective, less costly healthcare in principle and explaining how the United States -- with the embattled hard-won regulations we have on the books, the healthcare, insurance, and pharmaceutical industries that exist, and the actual diversity of stakeholders who do and will continue to have a say in the proposal and implementation of healthcare policy to come -- gets to single payer healthcare from where we are.
It is easily possible that a candidate who is NOT advocating for the repeal and replacement of the current healthcare system for a single-payer system can actually get us closer to single-payer by lowering costs (further incentivize prevention, encourage a shift from fee-for-service model, allow bargaining to lower prescription drug prices, among other things), coordinating public healthcare providers (Medicare, Medicaid, VA, Community Health Centers, Women's Health providers, say), and further expanding Medicare piecemeal (lower Medicaid eligibility threshold, offer voluntary early Medicare buy-in, and so on) in ways that will benefit more and more and more people in ways that elite-incumbent stakeholders will resist but eventually accommodate.
It is also easily possible that a candidate who IS advocating for single-payer could open up a political can of worms that provides reactionary opponents of the ACA an otherwise unavailable occasion to destroy the inadequate but actual incremental improvements in healthcare provision and access the Democratic Party finally managed to achieve after nearly a century of effort in this country.