There is also the theory that immigration has always been a part of the history of the United States while it hasn't in Europe, making integration of immigrants easier.
The best point of divergence would be the Romans never conquering Britain. Ireland was introduced to Christianity through its contact with Roman Britain. Irish missionaries converted Scotland.
Puerto Rico is relatively white because of a lot of Canarian settlement in the late 17th century and the 18th century and Catholic European immigration in the 19th century.