Mark I know zip about WFO, but my guess is that the materials are chosen to absorb heat while the logs are burning, and slowly release it when the logs have burned to embers, and have been moved out of the way to cook pizza. Any time you have a large thermal mass- you can store lots of heat and release it gradually. In contrast, if you can generate enough heat to cook the pizza all at once ( 2Stone, or Blackstone ) you don't need thermal mass, or even insulation, you just keeping pumping in enough heat to keep the oven at the temp you want. In my limited experience, the main reason for choice of deck is balancing top and bottom heat. If you made a metal deck, and heated the oven to 700, the whole oven, including the deck, would get to 700, and the metal would conduct heat to the bottom of the pizza so quickly, it would char before the top browned. In contrast, if you use a material which conducts the heat to the bottom of the pie less efficiently, you can get the oven to 700 and the bottom wouldn't burn as quickly. Just my .02.