The difference is that immigrants to the US melted their culture in with the existing one, creating the America we have today. Everyone adapted. People came to the USA to become...Americans. One can quibble about hyphens but at the end of the day it's always "-American"

Of course. That’s why there’s a Chinatown in almost every major city. Why LA has a Koreatown. Why Little Havana is such a big part of Miami. Why New York has a “Little Italy”. The Chamber of Commerce of those areas doubtless assigned those names so Real Americans™ could stop in and help them speak their English gooder.

My folks lived around Lancaster Pennsylvania, where German is still spoken in many stores and places more than 100 years later. In Chicago we had Polish cleaning ladies, who knew only a few words of English: “Mrs.” and “cash”. Mrs. Goofy comes from outside Philadelphia, where there is a suburb called “Germantown” where lots of people still speak, uh, German.

If you’d like a list of areas where your theory isn’t working, wikipedia has an pretty extensive one:

en.wikipedia.org - List of ethnic enclaves in North American cities