Our newest immigrants refuse to assimilate because they come here to work the system and have no respect for the ideals of the American dream. They reject the laws because they came here in defiance of the law and continue to have no respect for a nation that does not uphold their own laws.
The reason that they assimilated is that there are cultural similarities among the Europeans that cut across traditional national boundaries. In fact, those boundaries have not always been where they are currently. Germany and Italy were unified in the late 1900's. Austria-Hungary was broken up in 1918. The United Kingdom is a union of England, Scotland, Wales and Northern Ireland. Some ethnic groups like the Latvians and Estonians have usually not had nation states. Today, there are smaller ethnic groups in the Baltic area who do not have nation states.
Our newest immigrants refuse to assimilate because they come here to work the system and have no respect for the ideals of the American dream. They reject the laws because they came here in defiance of the law and continue to have no respect for a nation that does not uphold their own laws.
Europeans who come to the US still assimilate pretty well. The phenomena you cite mostly applies to immigrants from Latin America, and especially Mexicans in the southwest. Second generation Americans from most other areas are pretty fully assimilated unless they observe some weird religious orthodoxy.