The primary reason 99%+ of schools don't lock-in tuition is in the event of a year with especially high inflation, a bad economic hit (e.g., 9/11), etc. Also, there's something to be said about 3 people sitting in the same lecture and paying a different amount of money for the exact same service.
Say you run into an economic catastrophe or even a political catastrophe (e.g., Penn State). If inflation soars or demand plummets, you may have to raise tuition to make ends meet--maybe a significant increase--you don't want to be stuck with only being able to hike tuition on 25% of your student body.
The risk of locked-in tuition is just too great. And there is absolutely zero reward for the school. High risk, no reward is not a wise business decision. I appreciate TCU protecting its endowment by not making poor ones.