Bozette pointed out one of the reasons in a question I asked: the proximity to the Middle East. It's simply easier for Europeans to travel to Syria or Libya (the latter in the case of the Manchester bomber), become radicalized there, and return to Europe spreading the ISIS agenda (which often includes attacks).
Another reason is that the Muslim communities in Europe are far less assimilated than they are in the U.S. We seem to have a better track record with immigrants and allowing them to become and live as full integrated members of society at large. Stories I've read in France indicate that Muslims are often confined to "ghettos", they have high unemployment and poverty rates, and are shunned from European society, which causes them to be isolated and allow for more radicalization (ISIS specifically targets young Muslim men who feel disillusioned and disconnected from society. It gives them a cause to believe in and something to become a martyr for. There are simply fewer targets in the U.S., though it still happens).
France and England also once had colonies in Muslim lands (Algeria, Tunisia, Pakistan, etc.) which presents an even more complicated relationship between immigrants and the country they've emigrated to.
Benedict Arnold 