1. They are institutions of "indoctrination" by the other side. Faculty are something like 98% registered democrats and many subjects ("X studies") have an explicitly left-leaning bent.
2. They have tax advantages and other significant government subsidies.
3. They exercise significant amounts of ideological control over the narrative for their groups of people.
4. They are exclusionary of people outside the club.
Add to that the fact that universities are getting increasingly expensive and real life outcomes for college-educated people are getting worse. The perceived costs used to come with significant benefits, but the costs are getting higher and the benefits are reducing, so there is less tolerance for giving them favored status.
People with a more authoritarian bent view dissent itself as objectionable. That's central to their whole worldview. Any institution or social organization that allows debate or questioning things is a problem for them.