Because there’s no such thing as real pizza outside new york. So they have to substitute.
The best pizza I’ve ever had, and I live nowhere near there:
Leonardo’s Pizza
http://www.tripadvisor.com/Restaurant_Review-g37743-d4954353-Reviews-Leonardo_s_Pizza-Cedar_Rapids_Iowa.html
Iiked the pizzas I had the times I was in Chicago years ago. Never been to NY.