1. Demand. It is now commonplace for everyone in society to encourage all students to aspire to go to college. This simply was not the case decades ago. A high school education used to be much more valuable than it is today, and many people began their adult lives. Now, even marginal students are encouraged to at least give college a try. So just like with any other marketplace with a relatively stable supply (colleges are popping up everywhere) and increasing demand, there will be higher prices to maintain a stable situation.
2. Technology and quality instructors . Universities are spending money on the latest gadgets, equipment, and vying to offer students the best possible opportunities with faculty with higher credentials. Better teachers and learning environment costs more money. Similar to #1, many parents want "the best" for their child. Academic services, advisors, access to the latest fad, recreational opportunities, etc.
3. Federal government. There are expanded pell grants, loan programs that help people who otherwise wouldn't be able to afford school. This dovetails with #1 above. In addition, this guaranteed money allows schools the flexibility to spend more overall, which increases the value of what they offer students. Which leads to higher prices.
4. Business minded people working in college. If you could, wouldn't you try to get as much money for your job or office? In terms of salary, conferences, traveling, perks, etc? If people keep paying it why would you stop and cut back? Instead you would be incentivized to come up with great ideas that would absolutely be necessary for your department or the school in general.