[tenebrisalietum]

It's probably because of two reasons:

1. Most of northwest Indiana (Lake County) is considered part of Chicagoland, but is a different state. For example: Growing up there, I remember my parents crossing the state line on Sundays to buy alcohol, which was not legal to sell on that day in IN. But NWI had all the same radio and TV as Chicago.

2. Outside of of said Chicagoland, Illinois and Indiana are very different, especially the more south you go in both states.