Without meaning any sort of disrespect to games between Nebraska/Wisconsin, I don't think you'll find many Wisconsin fans that consider Nebraska a rival - most say Iowa and Minnesota. However, a larger-percentage consider Minnesota the primary rival and it extends beyond football, and athletics, in general. Few examples below:
- Tuition reciprocity allows for HS students in each state to attend Minnesota and Wisconsin at the cost of an in-state student. Kids, naturally, want to get away and this one way of doing so. For as many kids within each state going to their flagship university, many of their peers will cross state lines for university.
- Folks from each state find work in the other state after college in the major metropolitan areas (i.e. Minneapolis / St. Paul, Madison, Milwaukee).
- Both states have strong German and Scandinavian roots.
- Both states are similar in culture (i.e. recreation in the North Woods, access to 10,000+ lakes, etc.).
- The two states border each other, almost entirely, north/south. Plenty of crossover between the two states with residents working in the other (i.e. commuting from Hudson to MSP, Winona <-> La Crosse, etc).
- Minnesota and Wisconsin sports, not including CFB, are more-closely tied when you consider strong college hockey traditions (both men and women) and Packers/Vikings.
The rivalry between the two schools, for as lopsided as its been in football, is about as natural as it gets.