I think it has a bit more to do with heat distribution. Ordinary electric and gas ovens can have "hot spots" that are hotter than the rest of the oven. In fact, the professional slab-style pizza ovens are intentionally built this way - it's up to the oven tender to check the pizza frequently and make sure that it's baking evenly. It also makes it much easier to deal with thicker and thinner pizzas, or make a well-done pizza for a particularly discerning customer. (I like myself a well-done pizza now and then - lightly crispy crust, soft and chewy on the inside, and that lightly toasted cheese on top... YUM.)

A cast iron pan, though... Like G said, it acts like a heat sink. And what do heat sinks do? Well, they don't just absorb heat - they do at first - but once it's taken in all the heat it can (given ambient temperatures, of course)? They distribute that heat, all nice and even-like.

This is why convection ovens are such effective pizza-making machines. The convection action ensures the heat is evenly distributed throughout the oven. In fact, most any pizza place that has a conveyor-style oven is essentially just a really big convection oven on steroids - the one where I work cranks out a perfectly done pizza in five minutes. (Which is why many pizza delivery chains can make good on the promise of having your pizza to you in 35 minutes or less.)