Train service between MSP and Chicago is a perfect example of everything that's wrong with Amtrak. There's a single unreliable train each day between the cities and it lumbers along on a schedule that's 2+ hours longer than trains of 60 years ago! In comparison, the airlines have approximately 45 flights every day in each direction, so you can see this is a major travel corridor. In addition, Greyhound has introduced express service between MSP and Chicago with brand new buses equipped with free wi-fi, leather seating, and extra legroom. Greyhound's advance purchase online fares are only $24 one-way. I have taken Amtrak between MSP and Chicago in the past, but it's unlikely I will in the future. Why should I pay fares that are sometimes as high as an airline ticket or three times the cost of Greyhound only to find out the train is running hours late and I'll be put on a bus.
From what I understand, you can blame Governor Scott Walker. I believe the Wisconsin HSR funds that he turned down (and that my state, California, gladly accepted) included upgrades that would have made additional CHI-MSP frequencies possible.
That is not really true. The work associated with the proposed Madison train would have included upgrades of the Milwaukee and Watertown portion of the CP line used the Empire Builder. That is about 50 miles of the 330 mile route between MKE and MSP. The remaining 280 miles was not being touched. From Watertown west, the proposed route for the train to Madison was not on the line to Minneapolis, and those upgrades would have been of no immediate help for putting more trains between MKE and MSP.
The stated plan was to someday extend the Madison service on to Minneapolis, but that would be a neat trick since none of the existing rail routes out of Madison go directly to Minneapolis. At best, a route from Milwaukee to Minneapolis through Madison would be about 60 miles longer than the existing line, and the alignments of the existing lines are anything but "high speed." That is why the Empire Builder does not serve Madison today. Of course, for just a couple of tens of billion of dollars more, a new route could be established.
The sales pitch that the proposed Madison train was but the first step of a "high speed" rail line to Minneapolis was, by and large, vaporware.