Why do supervisors find it so difficult to provide honest feedback to their subordinates?

Few possibilities:
1. They've never had honest feedback given to themselves.

2. Good news is easy to deliver, but it's difficult to tell people what they are doing wrong. Especially, if they think they are doing a good job.

3. The importance of feedback has not been stressed by the member's chain.

4. They figure everyone gets a 5, so why bother.

IMHO, I think some supervisors have difficulty separating themselves from their subordinates as peers. Once you step over the threshold into management, you become a manager, not a peer. Some supervisors have a hard time making that transition, and would rather continue to be people pleasers. You can lead effectively without worrying about everyone liking you. In fact, my personal statement as a leader is, if I am pissing someone off, I must be doing my job.

Fear of losing a 'friend' or not being the popular NCO (if speaking strictly military).