WWI also forced the issue in a way that hasn't quite applied to other "ethnic" folks in the US. German immigrants, by and large, have been pushed towards forgetting their national culture altogether and assimilating into a newly-manufactured (by early 20th-century Progressives, no less), unified "White" identity.
There was as much (more, even) pressure on the Italians and Irish to do the same.
But, to add to and enforce OP's point, their cultures were a larger schism away and they instead held on and entrenched their identities (to their objective detriment, no matter your moral stance). Meanwhile, Anglo, Franco, German, Dutch, etc cultures all kind of melded into the early "White American" identity; due to the general ease of assimilation.
How much difference is there, really, between various European nationalities? They are all white and Christian, using similar language and script, mostly the difference is in the Christian denomination.
Similar language is a stretch. Sure, European languages have similar roots, if you go enough far back. But the ability to speak your native language with a fellow European of another nationality, and have a fluent conversation, is quite limited.
