This article provides a good explanation:
http://thefederalist.com/2017/07/11/heres-many-republicans-view-americas-colleges-universities-negatively/

"The simple answer to this question could stem from recent events at universities like Middlebury College or the Naval Academy. Student (and staff) protestors have, in many of these instances, refused to listen to speakers they disagree with—sometimes turning violent in the process. In some of these examples, professors have lost their jobs (or been mistreated in some fashion) for touting unpopular ideas."

"The debate over “safe spaces,” “trigger warnings,” and “political correctness” has risen to a head over the past couple years, as professors (and students) seek to cater to the wishes of their classroom—sometimes to the detriment of real learning, or even real belonging."

There's a lot more to it than the above. Conservatives also question the cost/benefit that students are getting from colleges etc... As explained in the article.