And when? From what I understand it became popular sometime before WWII and never really stopped, although it seems to have been first sold as "Pizza Pie", and dropped the "pie" at some indeterminate point?
I think the ancestor of American pizza is the tomato pie, popularized in early 20th century New York by a variety of Italian American entrepreneurs. I think it's popularity can be attributed to the readily available and cheap ingredients, which makes it very accessible to everyone.
Canned tomatoes, romano cheese, and flour are all shelf-stable, so this makes it easy to produce pretty much anywhere in the US. Once it established a foothold, it just snowballed out of control; because, well, pizza is delicious.
If anyone has reading suggestions on the subject, I'd be interested to know, first because food might be an interesting category of analysis and second because pizza is delicious.
If it is similar to the Australian reason, it is down to certain waves of immigrants arriving in the country at certain points in history, and taking up businesses selling hot food - in this case Italian. Fish and chip shops were traditionally started by Greek immigrants in Australia. Can't thank all of them enough!