Well, M4A and mixed private/public insurance systems are at odds with each other. Harris initally endorsed M4A & abolishing private health insurance. Just a week later her campaign stated that Harris actually would support any type of health care reform, including mixed systems, which are fundamentally at odds with M4A.
Of course her job as a Senator necessarily requires her to be more open minded to incrementalism, as she could simply be voting on others bills, but as the President, she would have far more ownership over the type of reform, so its very important for her to have a clear mandate.
The problem now isnt even necessarily her specific policy stance, its the fact that she so clearly doesn't have one, she is just vaguely hedging, so as not to be outflanked on the right or left.