The corridor of severe weather supportive parameters was very small. Southern Wisconsin and the eastern half of Iowa had the necessary ingredients to see severe weather. Those are the locations that witnessed a few severe thunderstorms. Areas such as Galena, Savanna, Freeport, Janesville, Monroe, and Sterling were on the fringe of the "best" area to see severe weather. The moisture content, instability, and wind shear were all much higher than those same parameters in Rockford, Belvidere, DeKalb, Sycamore, Rochelle, Oregon, Byron, and Amboy. It's not very common to see such a difference in atmospheric profiles in such a small area.
Since the parameters were not in place over much of northern Illinois, the thunderstorms died as they approached the Illinois-Wisconsin border Monday evening. Not only did much of the Stateline lack instability, shear, and moisture, but there was an atmospheric "cap" in place aloft. In other words, a warm layer existed thousands of feet off of Earth's surface. That warm layer results in what is referred to as a stable environment. Thunderstorms do not like stable environments. Thus, as storms approached northern Illinois (aside from Jo Daviess and northern Stephenson Counties) they quickly faded, essentially approaching a wall of dry air.