Iowa had/has a rivalry with Nebraska, so they play each other for the last game of the regular season.
Current week 13 matchups:
Indiana/Purdue
Ohio State/Michigan
Illinois/Northwestern
Iowa/Nebraska
Minnesota/Michigan State
Wisconsin/Penn State
The first four make sense, they are actual rivalries, the last two do not, but there does seem to be some reason for this. After a look at Wikipedia, it seems that if we played Wisconsin on the last game of the season, that would leave only Michigan State and Penn State, who are in different divisions and do not have a protected rivalry, and so do not play every year like Minnesota and Wisconsin do, so Minnesota and Wisconsin got stuck with teams within their respective divisions that they play every year. Still, I think Minnesota and Wisconsin is arguably the second biggest rivalry in the conference after Ohio State and Michigan, and I feel like it should have been played on the last week instead of Illinois/Northwestern.
There are three cross-divisional protected rivalries (and I mean actual rivalries, not just protected crossover games), Ohio State and Michigan, Illinois and Northwestern, and Minnesota and Wisconsin. So, it was likely pretty easy for them to put intra-divisional rivalries (Indiana/Purdue, Iowa/Nebraska) on the last week of the season, and they sure as hell weren't going to move division crossover Ohio State/Michigan off of the last week, leaving two crossover rivalries (Minnesota/Wisconsin, Illinois/Northwestern) which would likely both like to have their games played on the last week. At this point, Indiana, Purdue, Iowa, Nebraska, Ohio State, and Michigan all have opponents, and Minnesota, Wisconsin, Penn State, Michigan State, Illinois, and Northwestern do not. That is three teams from each division. Minnesota/Wisconsin is a protected crossover, and Illinois/Northwestern is a protected crossover, but Michigan State/Penn State is not. If Minnesota/Wisconsin and Illinois/Northwestern were to both be played on the last week of the season, that would leave only two teams, Michigan State and Penn State, without opponents, which would stick them with each other. Michigan State and Penn State are not from the same division and are not a protected crossover, and so they cannot end every season by playing each other, meaning only one of the two protected crossover rivalries (Minnesota/Wisconsin and Illinois/Northwestern) could be played on the last week. If they allowed us, Minnesota, to play Wisconsin on the last week, Illinois would end their season with Penn State, and Michigan State would end theirs with Northwestern.
Basically, under the current arrangement, for Minnesota to play Wisconsin on the last week, Illinois/Northwestern would have to be moved off the last week, or Ohio State/Michigan would have be moved off the last week (and we all know that's not going to happen). It had to be either Minnesota/Wisconsin or Illinois/Northwestern that didn't get to play each other on the last week, and our rivalry was the unlucky one chosen.
That said, with the current protected crossover games, Michigan State gets Indiana every year, and Penn State gets Nebraska. If they were to switch it to Penn State/Michigan State, and Indiana/Nebraska, then Michigan State could end their season with Penn State, and we could end ours with Wisconsin. Nebraska and Penn State are probably the best programs in the conference after Ohio State and Michigan, so I could see where having Nebraska get Indiana every year seems lame from the Nebraska point of view, and seems unfair from the Indiana point of view, but as now, Michigan State and Nebraska are probably going to be about equally difficult for Indiana to beat, and Michigan State/Penn State does actually have a trophy tied to it and would probably be a good game regardless.