My deceased great grandmother, an Italian immigrant to the US, was surprised that pizza was becoming a big thing here later in her life. She'd lived through a childhood of poverty in northern Italy, as well as the great depression in the US. In both situations, pizza and polenta were simply satisfying foods that could be made extremely cheaply.
When she'd moved here, Italians often faced discrimination, and, for instance, my grandfather (on the other side) refused to eat Italian food and otherwise looked down on Italians.
When I was growing up in the 80s, pizza was already a big thing. There were chain restaurants like Pizza Hut serving up the stuff, as well as an incredible amount of mom-and-pop stores, not to mention the big-name restaurants in places like New Haven.
But what got the nation from looking down on Pizza to having it be a cornerstone of "American Cuisine." that everyone loved?
You may want to crosspost this to /r/askfoodhistorians
You may be interested in this previous answer of mine. While it doesn't address your question directly, it's a timeline of pizza's popularization across the United States.
A TL;DR version would be: Pizza was introduced to America by Italian immigrants in New York City by 1903, and quite probably in the decade before that. It became a popular dish in New York City between the 1910s and 30s, spreading out to the greater metro area and then the Northeast, with pizza-by-the-slice introduced during this period.
With urbanization and the opening of Italian bakeries in other major cities, pizza became a popular dish across the country in the 1940s. By that decade, pizza recipes were regularly showing up in newspapers and, to some extent, cookbooks. Pizza delivery had also started in earnest by then, with ads dating back to at least 1935.
By the early 1950s, frozen pizza was available in supermarkets. An article in the January 23, 1958, edition of the New York Herald Tribune claimed that there were 20,000 pizzerias nationwide, and 100,000 outlets to buy pizza if including frozen and fresh pizza available to take-and-bake from supermarkets.
In 1960, the precursor to Domino's began in Michigan, with a business plan centered around delivering pizza to students on college campuses. By the mid-1970s, Domino's was a national chain, with Wichita-based Pizza Hut hot on their heels.
/u/Alieneater and I tag-teamed on an earlier answer, though I basically re-wrote it and cannibalized Alieneater's info for the answer linked above (just wanted to give a shout-out).
/u/Yodaracits wrote this answer earlier on, which adds even more information.