Well if they didn't want to be absorbed, they could have stayed home and saved the expense of the boat trip - except for the African population of course. But other than slavery no one was 'forced' to come here, they chose it - for whatever reason.
Many came here expressly to escape a lot of the aspects of their own diverse cultures. Which of course is not exactly right, they wanted to get out of their society without giving up all their culture. Russian immigrants didn't want to stop being Russian, but they sure as >-bleeped-< wanted to stop being serfs. And while it's always been fine to absorb/celebrate bits of culture, we expressly did not want to incorporate aspects of their home societies. So a little give, a little take - some of this, none of that...
And even then it's not universal. There exist several unique and separate societies and cultures in the US. The Amish and Mennonite communities, the Cajuns, and the entire Western Cholo/Atzalan deal are pretty separate, as are many of the native tribes.
And it's not forced by American society and culture, it's something that happens over time, it's generational, and often, it's done out of a mixing of nostalgia, pride, and belonging. It's not all that unique - quite common in fact - to go to an Indian neighborhood and see grandma walking in her sari, the daughter in more contemporary western garb but with the bindi, and the granddaughter in the same hip-hop stuff all her classmates wear. But though she is not going to be sporting a sari, or a bindi, she'll still like some aspects, music, food, faith - of the heritage culture, and she'll incorporate them into her life, and in fact, help spread them.
So now, back to Easter eggs... no doubt as the immigration with the tradition moved into their neighborhoods they were right up against other immigrant populations, they shared space in housing and probably more importantly in church. So when the Irish kids saw the Polish kids with the cool eggs they started doing it themselves, and no doubt the Polish people where happy to share it with them. Just like the Indian granddaughter, who has learned to cook some of the old country dishes will share them with her friends, or her friends will come over to dinner (or she'll just open a restaurant, one of the great American traditions, using your ethnicity to your advantage) and go 'Damn, that curri is good.' Or like all the people I know who do luminaria after seeing Hispanic houses do it.
