There is no one simple answer but the following have to be reasons and I apologize if I am repeating.
- Cuts in state funding for universities
- Availability of fed loans with no consequence to colleges
- Continuing increase in wealthy foreign students (China) in particular
- Never underestimate the desire of the wealthy to maintain access to privilege by making it un-affordable for everyone else.
- The slick marketing qualities of universities who use brand to sell the idea quality is determined by cost.
- Hiring practices in certain sectors of the economy