Two biggest drivers are 1) massive declines in public support (for public universities, obviously) and 2) more administrators.

Much of the growth in administration is driven by a significant rise in the costs to comply with federal regulation. Those regulations are not bad--it's the cost of complying with things like disability laws, Title IX, etc--but they require collecting and reporting significant amounts of data, and that isn't free.

There's other stuff too; some colleges do have the lazy rivers and fancy dorms, many colleges lose money on their football program, etc. But those aren't the fundamental drivers.

By the way, in case anyone is wondering, the money is definitely NOT going to faculty salaries. Salaries for full-time faculty have been stagnant for decades even though an increasing percentage of classes are taught by poorly-paid adjuncts.