I think there are a number of factors at play here. One of them is almost certainly the rugged terrain that makes agriculture profitable, but less so than most other agricultural areas. Outside of metro Madison, the driftless is quite poor.
Another factor is the Madison metro itself bleeding over into Rock, Green, Iowa, and Sauk Counties (but not the counties further out like Grant and Richland). The 2016 results demonstrate this pretty well.
Another factor is the different ethnic and religious background of the driftless area compared to much of the state. This map, which has a scaled display of religious groups as RGB values for each county (Catholic = green, mainline Prot = blue, evangelical = red; note that brighter = more of that group and that I used the ARDA database for these determinations, which most notably counts WELS and LCMS as 'evangelical') aligns well with the older election maps in western WI:
The driftless is less German and more Scandinavian than the rest of WI, poorer, and has worse terrain for agriculture, all of which contribute to its voting patterns.
Some other notes:
-The Driftless area is both very swingy and far more Republican in state legislative races than it is in federal races.
-This is an area that buys into the Law of Jante far morseso than almost anywhere else in the US and it affects how people there react to things. People will vote for whoever makes an argument that he or she will serve their interests and provide good government.
-In older times, the northwoods was also a democratic leaning area, and still is willing to vote for democrats occasionally, but has swung toward the GOP more recently. This swing accounts for most of the differences between the two maps Averoes posted. The driftless has sort of followed suit in a pattern from north to south.
-There are some similarities to Vermont, but make no mistake, the Driftless area does not particularly vote for moderate Republicans over any other type. Compared with New England, it is an entirely different breed. The Driftless is swingy for a rural area but not necessarily culturally liberal.
-The Driftless Area is notably anti-war and Trump's relative non-interventionism likely helped him there.
Interesting. Thanks for the info!
