Concentration (including numbers and physical proximity; Europe is far far denser than Dearborn which leads to a communal culture - look at the slums of Naples for example and what proximity breeds), and being forced to work. The unemployment rate in Dearborn, Michigan, a place without meaningful welfare, is 3.9%. The unemployment rate in Molenbeek, Belgium, where many jihadis live, is
40%. Welfare is the difference between those two numbers.
It's also far closer physically to the Middle East. Which creates stronger ties, more frequent trips, more visits by radical preachers.
The answers are all really really simple. Congratulating the US government/policy or US culture (apart from its individualsim/rejection of socialism and welfare) is silly, IMO. I think the US would cope far worse with the kind of numbers that Europe is.