greg, I suspect it's because the English ancestry goes so far back in the history of the US. People whose families go that far back tend to like the fact that they are of English ancestry, but they also are very proud of the fact that their families have been in "America" for so long. So it's usually the "American" history that they are more concerned with.

And immigrants from other countries who came here later were confronted with a national identity that was largely English, so they saw themselves as different, and many (although hardly all) groups of immigrants became what could be described as "cultural sub-groups".