I don’t think critical mass explains it. There are places in the US comparable to the Netherlands in population, density, and Muslim population (the New York Metropolitan Area has a population of ~19m to Netherlands’ ~18m, and ~4% Muslim to Netherlands’ 5%.
For a rapid increase in Muslim population, look at Minneapolis-Saint Paul, with a population about twice the size of Amerstam, ~10% Muslim to Amersterdam’s ~7%, almost all of whom arrived within about 30 years as refugees from Somalia.
I think your last point is strong, though: the US still has an active Christian religious culture, and I’d wager that even our most secular cities have more practicing Christians than most places in Europe. That would definitely affect how Muslim immigrants interact with society.
But I think more important is that the US is just more diverse in general. We’ve had a relatively permissive immigration system, and paired with birthright citizenship that means our ‘native’ population traces ancestry to lots of different places countries and cultures. We also tend to be less strongly identified with the city or state in which we were born: in Europe it seems common for a person to claim many generations of ancestry in an area the size of one of our smaller states; in the US more than a few generations is unusual, and for almost everyone there’s a sharp cutoff (even most Native Americans, as most tribes were displaced by European expansion).
Which goes back to something that I said earlier (not sure if this thread or another): Europe is becoming diverse in a way that the US has been diverse for a century, and the problems have more to do with other-ness than with the particularities of the other.
Yeah what kind of a secular liberal would defend religious freedom and oppose discrimination. Imagine…