Schaumburg Airport lies underneath some of the busiest airspace in the world and is 8 miles west of Chicago O'Hare, one of the most active airports in the US. It's not the busiest uncontrolled field in the US, but it gets more traffic than some towered airports. Given the location, airspace complexity, and airport popularity why is Schaumburg an uncontrolled field?
Most likely, the airport was formed in the early-mid part of the 20th century as a small country airport outside of the Chicago metro area. The city grew around it, similar to Compton-Woodley airport in Los Angeles or Cross Keys or Camden Co. in the Philadelphia metro area. With the introduction of the modern airspace systems for commercial air travel the Class B shelves covered these airports but did not include them.