Pizza travels well, is still good (maybe better) after it cools a bit (but still warm), and has enough volume demand for delivery.

Burgers are ok a bit cold but still warm, but fries are not.

A pizza place with a driver on staff can do route planning and coordination so maybe the pizzas for a group of three deliveries near each other come out of the oven around the same time, hopefully right as the driver gets back from their previous run.

That's harder to do with McDonald's. And if there's much of a wait, the fries show up cold and it's a bad experience.

Most pizza places that do delivery have inhouse full-time pizza delivery drivers. The margins on pizza is so large, delivery can be done at cost (+ tips) and the pizza store can still come out on top.