Apart from the political opposition from the insurance companies that Medicare for All would have engendered, I think the main justification for abandoning Medicare for All and switching to the PO and eventually the PO-less ACA, was actually neoliberalism. The President, his main advisers, the Democratic leaders in Congress, and most progressives working for Washington progressive organizations were steeped in neoliberal doctrine. They viewed the Bush tax cuts and the two Wars as unpaid for. The ARRA stimulus Act was similarly unpaid for and added to deficit spending and to the debt-subject-to-the-limit. They believed and most believe today that the Federal Government can have solvency problems if the debt-to-GDP ratio increases too much, and interest rates on the national debt are driven up by the bond vigilantes.
A Medicare for All Act would have required Federal spending on health care to rise by $800 - $900 Billion per year over present levels. They were not ready to cover that with higher tax revenues, and they were not ready to deficit spend it because they viewed that as fiscally irresponsible, and believed then and still believe now that it's necessary to decrease the debt-to-GDP ratio over time.
So, they wouldn't consider spending for Medicare for All. They wouldn't look seriously at the hundreds of thousands of lives they were consigning to oblivion, at the bankruptcies and divorces they could prevent, or at the obvious fact that while HR 676 would have cost the Government $900 Billion more in money annually that the Government can create at will and at zero real cost; it would have saved the people who have to pay for health insurance, and health care out of pocket and in the form of “co-pays” $1.8 Trillion annually, thus providing a marvelous boost to the economy. Instead, they just said to everybody, that it was impractical and that the United States couldn't afford it; but that it would be able to to afford a self-supporting PO bill, and later when that was taken off the table, a deficit neutral insurance bailout like the ACA.