The question is: Why are Wisconsin roads superior to Illinois roads. Iowa and Indiana too.
Those three have much more rolling ground for better moisture runoff. That’s important. And IL ranks 3rd in the nation, behind TX and CA, for interstate highway miles.
But when it comes to square miles of the state per mile of Interstate road, IL has them beat bigly. ( I’ve trucked every mile of it except the I-90 Skyway in Chicago.)