Because there no mechanism or market force which acts to keep tuition reasonable. Demand is pretty inelastic as we've seen. No matter what the cost people still pay because the current system all but demands a college education. Highschool guidance counselors and parents have convinced everyone that a degree from a top program guarantees the good life. 18 year olds have access to near unlimited debt, and the school isn't on the hook when the loan goes breasts up. Due to this combination of factors, universities can basically charge whatever they want risk free. Given this scenario, why wouldn't you charge $80,000 per year?
Build another dorm, remodel everything, new gym, everyone gets an iPad. Spend, spend, spend if you can attract a few more students. Its the perfect racket.