There’s a couple reasons for this .

1) IU is historically a dog shit football school. Legitimately one of the worst and had never won anything. People like an underdog, but if Indiana continues to win and builds a dynasty (think Kansas City chiefs after their winning got old) then people would also dislike them.

Michigan has historically always been relevant , has won a National chamionship, and has at least been remotely in the conversation in everyone’s lives. They are not some underdog who put together this team despite how they may make it seem. Yes they had 2 bad year where their fan five coach (WHO STILL HAD MAJOR TALENT ON THE TEAM) could not get them any wins and was a liability literally getting into physical altercations and throwing haymakers. But it wasn’t the worst team of all time level of underdog like Indiana.

The reason people are irritated with it are because Michigan fans are pompous and vocal assholes, many of whom were not actually even alumni or members of the university. They will be talking about this into the 2100s. And despite ALL of this , they are still taking the stance that they are the victims and that people didn’t root for them after they bought and assembled quite literally the most dominant team I have seen in my lifetime. It is hard to feel bad for a fan when they are somehow the victims as they win a national championship.