It's pretty simple but there are so many variations to it. Even the same basic ingredients can produce a pretty different result. Some people like pineapples, some don't. But no matter what we put atop our pizzas, most people wherever they're from appreciate some type of pizza.
Few foods have such universal appeal. Tastes can vary so much by culture so why has pizza become so prevalent? I'd argue it's because of the simplicity and versatility of it.
Simple foods tend to be found everywhere. Bread, fried chicken, burgers, they all are pretty popular in most cultures. Pizza at its core is a flat piece of dough topped with tomato sauce and cheese. Whatever else you add is extra. But those 3 simple ingredients easily translate across cultures. You can use local variations of each ingredient but the mixture of all three just works.
The simplicity naturally lends itself to further customization. You can add almost anything to a basic pizza and someone somewhere will like it. It allows every culture to add their personal favourites to it.
Few foods have these qualities and I think it's key to its universality. The foods that become globally popular have universal appeal but also can be tweaked to local tastes. Pizza does it better than anything else.