I would presume that introducing a new item on menu is a big deal for the chain restaurants. It has to be made available in all locations at the same quality. If egg topping is to be introduced, they need to deal with supply chain issues, like finding a supplier of huge number of eggs of the same type and quality and transporting to all their chain stores in the country and also meet food safety standards and do all this reliably all seasons of the year, year after year. I think this would be a big investment and worth only if they are sure that there is a definite market for egg topping.
The Pizzeria operating on the XXth/rd street doesn't have this limitation and hence can try out more fancy stuff