Catherine Rampell mostly blames funding cuts, at least when it comes to public schools:
Every recession, states face a budget squeeze as their tax revenue falls and demand for their services rises. They have to cut something, and higher education is often a prime target. Why? Struggling states have to prioritize other mandatory spending, like Medicaid. Higher education usually falls under the "discretionary spending" part of the budget — and in fact is often one of the biggest programs, if not the biggest, in the discretionary category. State legislators also know colleges have other sources of funds to turn to.