Oh, hey, something I can respond to! To clarify, I'm a third year Health Care Administration major, so my answer will be to the best of my ability.

National health insurance actually began in Prussia in 1888 under Otto von Bismarck. This was done partially to ensure that he had a healthy army, and also to appease those decrying poor social conditions. Prussia was able to implement state-run medicine because it was a very centralized government. The US, especially before WWI, was much more decentralized, and the federal government simply didn't have the influence, the money, or the incentive to roll out such a massive undertaking. When World War I broke out, nationalized health care began to be demonized as a "Prussian menace inconsistent with American values" (Starr, 1982). It's around this time that the term "socialized medicine" began to be used in a derogatory fashion.

As for who opposed socialized care, the American Medical Association (AMA, no, not that AMA) was one of the most vocal detractors of nationalized health care. Seeing it as a threat to medicine as a private practice and a business, the AMA worked to kill several potential laws that during FDR's presidency that would have implemented a nationalized health care system beyond Social Security (Shi, Singh, 2017). When Truman ran on a campaign to ensure a nationalized health care system in 1948, the AMA ran the most expensive lobbying campaign up to that point comparing nationalized health care to communism. You can guess how well people took to anything related to communism in the 40's.

Another factor that entrenched private health care in America was indirectly due to WWII. When Congress passed wage freezes to prevent hyperinflation, businesses began offering health insurance as compensation for lost wages and as a workaround the law. When the Supreme Court upheld in 1948 that this was a legitimate part of union-business negotiations, the American health insurance landscape was very firmly entrenched in the private sector. From here on out, most citizens polled said they preferred their employer being their source of insurance. In 1999, only 18% said that they would prefer to rely on the government rather than their employer for insurance(Commonwealth Fund, 2000).

Hope that helps some. If I can answer any questions, I'll do so as well as I can. AMA (yeah, that AMA, this time).

Sources:

Commonwealth Fund. 1999 National Survey of Workers' Health Insurance. New York: Commonwealth Fund, 2000.

Shi L., Singh D., Essentials of the U.S. Health Care System. Jones & Bartlett Learning, 2017

Starr P. The Social Transformation of American Medicine. Cambridge, MA: Basic Books, 1982

Conversely, what made Americans so receptive to the concept of Medicare, which everyone seems to love?

It was mostly due to lobbying from groups such as the American Medical Association who led a campaign in the 1940s against President Truman’s proposed national healthcare. The debate over socialized healthcare during most of the 20th century must also be viewed within the context of the Cold War and anti-communist fervor in the United States. There was concern, particularly among conservatives, that socialized healthcare would lead the U.S on a slippery slope towards communism. Beyond fears of communism, some also just opposed the idea ideologically and thought that the free market was the best way to administer health care (just as many conservatives do now). During the 1960s and 1970s the cost of health care was not as significant as it is now and more Americans were unionized and received better healthcare benefits. Thus, there was also less agitation from the public for socialized healthcare. As we moved into the late 20th century and the present, fewer Americans have strong healthcare benefits and the cost of healthcare continues to rise while real wages have remained fairly stagnant. This has created new calls for socialized medicine.