Multiculturalism did exist across America and Europe for many centuries. Regional differences can be huge and different places have wildly different cultures.
For the most part, all of those other cultures were variations of the same root culture: European culture. You had some outliers, such as Africans and Chinese, but the vast majority of immigrant cultures in America came from Europe and were thus more compatible with each other than, say, Indian immigrants. All the Little Italy and Germantown immigrant neighborhoods served as buffer zones for immigrants to learn to integrate into American culture without it being too overwhelming, because they had other people there who spoke their native language and shared direct cultural ties, but had been there longer and could show the newcomers the ropes. But at the end of the day they had to assimilate to American culture, and part of their culture eventually made it into the wider American culture. For example, the Irish were prolific in American law enforcement, especially in East Coast cities like New York and Boston, and so their cultural traditions spread to the wider law enforcement traditions. That is what was really meant by America being a "melting pot". But when you start introducing cultures that are entirely alien incompatible with the host culture, things start to break down. There are just too many differences to be overcome, and cultural competition begins.
When America took global power after WW2 (as the last man standing by being difficult to attack, not by being the noble heroes who saved us all.)
This is a disingenuous argument. It wasn't because America was the last man standing due to geographic distance. It was because America was an industrial powerhouse. We were not only arming and feeding our own troops, but also our allies because much of their own manufacturing industry had been bombed. German POWs brought to America had their whole ideological world turned upside down simply by witnessing with their own eyes the living standards of the average American during wartime, which was much higher than the average German citizen BEFORE the war. German POWs were getting more meat with each MEAL in America than they would get for an entire week's rations in the Wehrmacht. Nazi propaganda had taught them all that American industry barely functioned and would fall apart trying to meet wartime demands of just America. Then the work release programs started and we began using German POWs to offset our labor needs, and they got to see firsthand our industrial might and that our production was not just for our military, but also the Brits, the Soviets, and our other allies. German POWs who were used to work farms got to meet farmers sons who drove tractors that the German farmers could only dream of owning back home. On the trains crossing the nation to go to the POW camps, the Germans saw rural towns in the middle of nowhere that had electric lights burning in every shop, house, and factory while back home nearly 80% of rural German towns still didn't have electricity. At the factories they saw rows and rows of privately owned cars. These German POWs would return to Germany to rebuild after the war, wishing for their country to have the prosperity that Americans had during the war. It wasn't America being far away and hard to attack that brought us victory, it was our industrial might that allowed us to build more bullets, weapons, bombs, ships, planes, artillery, radios, radar, uniforms, and food than the enemy could, and do so to such an excess that we were also supplying our allies. Being difficult to attack was definitely a benefit, but it didn't win the war for us.