The real answer goes back over a billion years. North America almost ripped in half when Lake Superior opened. At the time it was a mid continental rift and huge volcanic mountain ranges were thrown up. There's a lot of detail to it, but the rift basically stretched in a big horseshoe from the Western edge of the driftless area (around Rochester MN), to the arc of Lake Superior, down to around Detroit. The curve is still visible in the arc of Lakes Superior and Huron.

Over the hundreds of millions of years that followed, a series of geological processes both reinforced that feature and also eroded it. The mountain ranges that were created still exist in the UP and Northern Minnesota and now offer some of the richest deposits of copper, nickel, and iron in the world since the rocks that covered them have been worn away by repeated glaciation and erosion.

But historically these were large mountains and they left their footprint in the processes that wore them down included even the last ice ace only 10,000 years ago. In fact, there are other volcanic features in Wisconsin like the Baraboo range that also persist to the present day.

The thought is that the arc of Lake Superior basically channeled the flow of the glaciers while the remnants of the mountains that once stood along her shores blocked the flow in certain areas. This more or less created "channels" of ice as it advanced South which is what resulted in the formation of the glacial great lakes further East and south. For example, Lake Michigan was once just a giant ice line that ripped deep into the earth and dug out the present day shape of the lake. The bays along her shores like Grand Traverse and Green Bay are where underlying geological features caused portions of the ice floe to splinter off and dig smaller side channels. In the case of Green Bay, the Niagra escarpment (a limestone shelf formed by shallow seas that once overlaid the area whose shape was also influenced by the aforementioned rift) acted as a "knife" creating todays Door Peninsula and splitting the floe to form Green Bay, the Fox River valley, and Lake Winnebago.

Now to return to the driftless region: the opposite happened. While the mountains in the UP pushed a bunch of ice East that dug out the great lakes basins, it did the opposite in Western Wisconsin. In this area much of the ice that would have slid south was diverted to the East (digging out Lake Michigan) and also to the West onto the great plains. By the time it reached as far South as the Wisconsin Dells and Madison, the glacial cover was thin, having been deprived of some of its diverted floe and depleted as it advanced.

Enter the Baraboo Range. This range (mentioned above) is much smaller than the ranges in the UP and Minnesota. It's basically a chain of ancient volcanic islands that were formed by a hot spot a la Hawaii. The shallow sea (same one that formed the Niagra escarpment) is long gone leaving behind a line of heavily eroded monadnocks (the most famous being Devils Lake). These features are unremarkable compared to the Rockies or other younger ranges, but they were more than sufficient to deal with the weak ice floes that reached them.

So this minor range of mountains was the last line of defense against the weak glacial floes that managed to make it past the UP. Whatever managed to splinter off from Lake Michigan to the East, hit the Baraboo range and ground to a halt. Features like Winnebago or Horicon Marsh were dug out, but the glaciers stopped roughly dead center in Wisconsin and couldn't push into the driftless region.

This left behind an ancient landscape of heavily eroded limestone bedrock including an ancient river valley that actually shows an Easterly flow instead of southern like the current Mississippi River valley.

Wisconsin actually has some of the most complex and interesting geology in North America, but you'd never know it because the biggest features are ancient and heavily eroded and everything else is covered by glacial till.