What has happened over the last 20 years is an unprecedented separation of the parties along urban and rural lines. Look at this map from the Chicago Tribune of the last 6 presidential elections in IL.
The drain of Dems from the rural counties into the Chicago metro area and vice verse for the Pubs is obvious. When Dems were spread throughout the rural US a swing in popular vote would swing enough EVs so that the PV and EV tracked each other for over 100 years until 2000. Now that the Dems are so highly concentrated it's much easier to swing a bunch of EVs without swinging the PV as much. That's what happened this year - large concentrations of Dems in CA and NY weren't matched by large numbers of Pubs in TX so those Dem concentrations didn't convert into EVs.
The Dem strength in the Chicago suburbs in 2008 and 2012 was primarily a favorite son effect for Obama, and in 2016 was due to Trump's high negatives with suburban professionals. If Republicans had nominated someone else in 2016, they would've won those counties back, though not by much.