No it wasn't. Famously so. America was a melting pot.

In the 1920s when this happened 15% of the population was immigrants. IE first generation Americans. With backgrounds from all over. Primarily European countries, but not the ones you think. Russia, for example, was a major portion of that number.

America at the time was way more heterogenous than it is today.

A major portion of that homogenation happened due to 1950s era racism and redlining which turned neighborhoods from mixed cultures into homogenous cultures.

War did a lot.

My ancestors spent three generations in America speaking German until WWI made being too German something you didn't want to be.

I imagine a lot of Russian/Eastern European-derived-Americans felt a lot of pressure in the 1950s & 60s to be as generically "American" as possible.

Better communication and mobility does a lot. Maintaining the home country’s language in the second generation these days is hard. I know some kids who speak their parent’s native tongue poorly and some not at all. When they speak it well, it’s because of constant effort by the parent. Maintaining it in the third generation is extremely difficult and often doesn’t happen.

It’s similar for other aspects of culture. No matter where you’re from, I bet your grandchildren if not your children are going to celebrate Christmas in some capacity.

Many of the highest crime places on earth are ethnically homogenous. Culture is not determined by skin color. Rather, Japan places a high value on the rights of and responsibility to the collective good. People in the US are culturally much less willing to suppress any rights of the individual, even if it supports a collective good. You can be much more confident that others will do what's in your best interest if society will come down on them hard if they don't.