The public health answer: They don’t want sick kids not to have access to health care.
The cynical answer: They are finding ways to add to the bill.
It has nothing to do with ACA. There would be no need for ACA to require it because a) children can stay on parents’ health plan until age 26, and b) If the parents don’t have a health plan, students can buy their own plan from their state exchange or the federal exchange. It would be very cheap for a single 18-year-old.
https://www.nytimes.com/2017/07/07/well/family/do-college-students-need-the-campus-health-plan.html?_r=0