FWIW, it's true that "many" offer both. Yet, more to the point, the vast majority offer thin-crust pizza, while only a minority offer deep-dish pizza. That's not a coincidence. Heck, there are entire neighborhoods in the city where no pizza joint sells any deep-dish pizza. In other neighborhoods, only a single joint sells deep-dish (in addition to thin-crust), and it's extremely unlikely that the deep-dish sales from that single store > its thin-crust sales + the sales from every other store combined.