To be fair, an awful lot of upper-middle class esp. North Side Chicagoans aren't from Chicago. The inner suburbs are far more Chicago than a lot of neighborhoods in the city that are filled with various Midwestern emigrants (who didn't think they were good enough for NY, and didn't think they were good looking enough for California, so ended up in the second city.) The inner suburbs are filled with the descendants of white flight from the 50s-70s, who had been in the city for generations when most of its cultural touchstones were established.

I'm from Chicago and I like deep dish, but these big thin slices are also good. It's obvious what the default is: when I was a kid "pizza" meant Giordano's or Eduardo's or Lou Malnati's or The Medici, or Unos or Dues, etc. Thin crust was called "thin crust." This article uses the same convention.

edit: I think there was an exception for Reggio's. Maybe we thought of it as "pizza" because it's so dense and advertised as a "butter crust."