I think, binding this sentiment with the other post someone made about IQ being heritable, it's clear what happened.
The settlers were people with grit and willing to take risks for their freedom. The voyage was tough as was colonizing a new land. They had offspring willing to make it work. The people left in Britain were the complaint sheep, and their offspring remain that way.
The immigrants that came to the US in the 1800s and early 1900s had grit and were willing to transplant themselves from across the world on a dangerous voyage in order to try and improve their life. The offspring of the Irish, Italians, and many German and Slavic migrants had these qualities as well. This was a frontier land. They had no plan of ever returning, or sending their money home. They were sick if monarchs and retarded Euro wars.
Today, there is no frontier and there is no risk. These are not people taking a risk, they are being trafficked into our country. They are not possessed of any grit. They're people looking for an easy time of it and a quick buck. They send their money to their family back home via banks. They all must go back.