Unbeknownst to Fab, some who came to this country were forced to assimilate or essentially killed, slaves. Those already here, Native Americans, often suffered the same fate. Once owning and killing of those who didn't assimilate became illegal, those wanting to assimilate weren't always allowed. Turns out some people didn't want them here, hence they stuck to their own and maintained their own cultures. This partly explains why certain ethnic groups dominated certain trades in NY, why black colleges and fraternities exist, and why Miami is predominantly Hispanic. The forming of these groups if you will only slows down assimilation.