Pizza is the favorite food of Ashford Academy because Pizza Hut Japan was a sponsor of “Code Geass.” Anime cross-promotion is a very common tactic for Pizza Hut Japan, according to YouTuber Red Bard, who documented some of the crazier campaigns.
The first anime Pizza Hut sponsored was “Ghost in the Shell 2: Innocence,” and they were given special thanks in the credits along with KFC. The Hut has done promotions with “Fullmetal Alchemist,” “Attack on Titan,” and yes, “Code Geass.” It has become fandom lore that Pizza Hut only paid for 3-4 logos in the show. But the animators thought it would be funny to include it everywhere, all the time. And thus a meme was born.
Pizza Hut Japan has diverged wildly from its American parent company. They offer shrimp, mayonnaise, and chili threads as toppings, and even pigs in a blanket crust.