Yeah, we know, pizza originated in Naples, where it was street food for poor people. It's also true that Margherita of Savoy made it acceptable in other parts of Italy, although it was still not popular. But it was the US more than any other country that made pizza what it is today, and fairly recently in the grand scheme of things. Before 1880, most Italians who immigrated to the US were from northern Italy, and did not eat pizza. Four million southern Italians arrived between 1880 and 1910, but their pizza still wasn't popular.
The first successful pizza restaurant in the world located outside of Naples was founded in Buenos Aires in 1882, when a Neapolitan immigrant baker named Nicolas Vaccarezza started selling the pies out of his shop in Boca. For reference purposes, a decade earlier, an attempt to open a pizzeria in Rome, Italy, had ended in bankruptcy, meaning, at the turn of the last century, you could get a pizza in Buenos Aries, São Paulo or New York, but not in Rome, Florence or Venice.
Only after World War II did pizza take off, as American entrepreneurs invested in pizza ovens, diverse toppings, delivery, and entertainment for diners. Now pizza is readily available in Italy, mainly because American tourists expect it. Read how all that happened at An Eccentric Culinary History. -Thanks, H.D.!