Good question, since if any one ingredient can be said to be unique to Chicago pizza, it’s corn (or rather corn meal). How did it get to be that way? Well my own guess is that it started out as a peel lubricant. Like traditional Italian semolina, corn meal has a particle size big enough so that when it’s sprinkled on a pizza peel, pizzas slide off of it like the bottom of your shoe over your kid’s marble collection (how many times have I told you not to leave these here????). Very handy for sliding pizzas into an oven. The problem with semolina is that it’s rare in the US which makes it expensive. Corn meal does the same thing and is both cheap and availble — what’s not to love?
However unlike semolina it’s not completely neutral tasting. It tastes like, er…corn. My guess is that at one point or another in the evolution of Chicago pizza, somebody decided they liked the taste, and incorporated a little of it right into the dough. Like potato starch it would have been a very effective “gluten interruptor”, giving a crust a unique flavor and texture (it also gives Chicago crust, along with a little vegetable oil, its uniquely yellow hue).
And what could be more fitting for a pizza invented in Chicago anyway? Chicago is the capital of the corn belt — the “City in a Garden” as it were. The inclusion of a proportion of corn meal in its famous deep-dish crust is only apropos.