What was the line from Stripes - ". . . our forefathers were kicked out of every decent country in the world?" One way or another, most Merkans' ancestors were displaced persons; they left the countries of their birth, but they took a little of that country with them wherever they went. Many times, they had relatives where they were heading, or settled in communities that already had a significant population of their particular ethnicity. The great American Melting Pot is more like a stew, and a lot of people like to remember who they are (or who they wish they were, lol) at least once a year. Everybody's Irish on St. Paddy's Day, right?
(And while we're talking ethnic celebrations, my parents enjoy heading up to Holland, Michigan, to celebrate my father's Dutch heritage during their Tulip Festival)