- inexpensive - delicious - stacks well (can serve at scale) - not messy, even without plates
Nearly everyone likes pizza, and there are lots of varieties for various tastes.
At my work, we've tried various foods, but we always come back to pizza because it's so universally liked and so convenient.
But I am more curious about culture as well. Pizza is very ubiquitous, and from my experience people prefer it even when they can easily choose something else to eat. Of course, many people just like pizza, but it is sort of a no-brainer to order pizza if team stays late, although it is simple to order something else.
edit: bad grammar :\
The history of pizza in the U.S. is itself fascinating. It was largely limited to Italian immigrants until WW2, when soldiers returning from the Italian campaign developed a taste for it. It exploded in popularity afterwards, particularly from the 1960s onwards. This is perhaps related to it being easy to eat without utensils, which made it convenient in the car/drive-in culture of that time period.
I was also surprised to learn that the vast majority of pizza varieties in the U.S. are actually American inventions, and that pizza in Italy basically consists of two varieties: marinara and margherita. All this stuff about pesto pizzas, BBQ chicken pizzas, meat lover's pizza, broccoli pizza, Hawaiian pizza, etc. are North American (including Canadian) inventions.
I wouldn't be surprised if it'll be replaced in a few generations or so by something else (popplers?).
It's a good question though!
It's pretty hard to mess up a pizza, especially if one of the main foods you deliver is pizza.
Plus, if you're a veggie/vegan, or you have gluten intolerances, pizza is still an option - as long as they get it right. I once had a nightmare scenario where a pizza place gave a vegan pizza to one of my co-workers with coeliac disease. When this person fell ill, the chef said "I thought they were just fussy".