I’ve noticed the same thing. I think it’s due to a few major factors. First, more Americans are becoming “foodies” and more adventurous in what they are willing to try. They’re becoming more accustomed to flavors that weren’t part of the American/European palate 50 years ago. So immigrant chefs can stick to what they used to make back home and locals will still go there. Second, there are more and bigger immigrant communities and in more cities. It used to be that Chinatowns, Little Italy and so on we’re only in major cities like San Francisco and NYC, but there are immigrant enclaves in every small city too now, so they have a reliable clientele of others from their home country.

The third major factor is availability of more high quality ingredients. New York style pizza developed because Italian immigrants couldn’t get the same cheese and fresh tomatoes they had back home. So they switched to mass produced low moisture mozzarella instead of fresh, and came up with a new style which was no less delicious and legendary, but still different. Today, nearly any ingredient on Earth can be sourced and either made locally or shipped anywhere in America for a reasonable price. The Syrian restaurant I mentioned has the best hummus and baba ghanoush because they can source tahini from Syria or another Arabic supplier directly, and it’s much higher quality than what you can buy in American grocery stores.