For cases like New Haven Style pizza and some of the original NYC pizza places like lombardi's, they use coal ovens which get really really hot but don't really have a flame like wood oven. So you end up cooking them longer than in a wood oven, which makes them crispier but also a little charred around the edges.

Given that these places served a lot of immigrants when they opened, I also wouldn't be surprised if it helped having a sturdier pizza that you could take to work with you.