Then you haven't investigated the matter very closely. Sanders has detailed plans and explanations regarding to how to pay for M4A. For the most part (exception: proposed capital gains tax) these would be unlikely to face any honest political issues from the Democratic side at least. (Yes, of course they would be non-starters from the GOP side since they involve taxes and increased public services). The fact of the matter is that the US taxpayer is already bearing the financial burden that would be potentially incurred for M4A in the form of premiums, deductibles, extra administrative costs, and employer-side expenses.
The bitter pill from a political standpoint is that M4A necessarily involves driving existing insurance companies out of business - many of whom already have powerful lobbies, donate significantly to both parties, and employ hundreds of thousands of people. (Some other prominent health-related industries have vested interests in M4A not passing as well; pharmaceutical companies, for example). And the reluctance of the American public to get rid of their existing health care without having a clear idea of what they would get in return.