[hatberet]

That might work at some universities, but there are a (not insignificant) number of students who simply refuse to buy used textbooks. After all, they aren't paying $20k per semester to learn old calculus!

The sad reality of the situation is that a professor's student evaluations (which do matter for things like tenure) will be negatively affected by the decision to use an old edition. Yes, sometimes it makes sense to tell an unprofitable/annoying customer to go away, but professors don't have that luxury in the university environment.

Edit: Curious why this is being down-voted. The truth hurts?