Doesn't that link suggest that it's primarily the mass, not the material? Steel is just an iron alloy after all, I don't think its thermal properties are much different (but 15lbs of steel will transfer more heat to your pizza than 10lbs of cast iron). Also cast iron can certainly crack, if it's dropped on a hard surface, or thermally shocked (but it is tougher than most ceramics).

From the article "The idea is that all else being equal—same recipe, same oven temperature—because of its superior thermal qualities (higher volumetric heat capacity, as well as higher conductivity than stone), you can cook pizzas faster than you'd be able to with a regular stone."

So it is thermal capacity plus conductivity together.