at 90% off they are not only selling it way below their own cost, but using up space that newer, full priced merchandise coul occupy. 75% off is also likely below their cost, and you said it yourself, it didn't sell because people were waiting for an even further markdown. maybe next time they won't wait.

Additionally, in some states there are some specific guidelines, tax implications, etc that make donating items more time consuming than it should be. The sad fact is that it is more cost effective to throw away.