2016 was a combination of the suburbs reacting poorly to Trump and the bottom falling out downstate slightly less than would have been expected, probably helped by the relatively unpopular Mark Kirk's underperformance there in the concurrent Senate race. The trends in 2020 were typical of the nation at large, with the downstate R trend strong enough to blunt Chicagoland somewhat and a much more nationalized Senate race against the heavily entrenched Durbin. In some downstate counties such as Fulton, most of the 2016 third-party vote went for Trump rather than Biden, and college students being at home hurt the Dems' raw vote totals in college counties like Champaign and Jackson.
Although that may make sense in theory, in practice it's not true in this case.
Champaign County saw more raw votes cast in 2020, and Biden's margin expand by 4.5%. In fact, Biden outperformed Barack Obama's performance in both 2008 and 2012. Biden's 60% in the county is the most ever for any Democrat. In no way whatsoever did it trend rightward in 2020.
Jackson County, though, makes more sense, since it had pretty much the same raw votes in 2020, and Trump's margin of loss went down from 3.2% to 1.3%.
