Not so. My county West of Chicago is decidedly Republican as are many areas. The problem is Cook County/Chicago and a few other areas have too much power over state machinery. And the Republicans have had a dismal record lately of hosting viable conservative candidates for state offices.
The last “conservative” that IL had was Dirksen unless you count Peter Fitzgerald’s one-term bailout. And Dirken would sell out conservatives just to get on LBJ’s good sides. The funniest thing he ever did though was shake his finger at Thomas E. Dewey at the 1952 convention. That was rare IL passion.