I agree it would be helpful if we could give a reason when choosing Reject and Edit, but the interface provides only a frustratingly limited feedback-giving mechanism even for Reject, so I don't think giving feedback to suggesters is its priority. In addition to allowing us to give more feedback, I think that when people get any kind of edit suggestion rejected through the queue (not when it just conflicts with another edit, which generally happens by accident), they should get a notification about it so they actually see the feedback.
When we reject an edit we are forced to give a reason or to edit the post ourselves. I believe this is less oriented towards educating new editors and more towards educating new reviewers and making us think twice before idly rejecting less-than-100%-awesome edit suggestions. To me it seems the UI is trying to get you to approve/improve if the edit is any use at all, or fix the post yourself if you don't like it. This is a good orientation, at least in the short term, because its outcome is that posts are likely to get fixed. Also, while flawless work can't be expected from all edit suggesters (new users and anonymous visitors are allowed to suggest edits), reviewers need to uphold a higher standard.
Still, it's frustrating that the UI is so limited in providing feedback. Even though we are forced to choose a reason through Reject, there is almost nothing prompting the suggester to review that feedback. They have to go to their profile, look under all actions, select suggestions and go to the rejected edit itself to see why it was rejected. The only indications they get that they can or should do that are
- the non-arrival of +2 reputation, which they might not even notice if they made many suggestions or had other reputation events, and won't apply at all if they've already earned 1000 points from edit suggestions
- a tooltip when they try to suggest an edit, telling them some of their recent edit suggestions were rejected and they should review them
- an edit ban (not sure what that looks like)
It's therefore likely that many people who suggest edits don't check back to see if they have any feedback at all. I don't think the process does enough to educate new editors, and due to this, some poor edit-suggesters who had their suggestions charitably improved, wrongly approved, or who ignored rejected edit feedback, keep editing poorly, and go on to review poorly when they earn the edit review privilege.
Although reviewers don't get to give a reason, users who check their edit suggestions will see this message as feedback when a reviewer chose Reject and Edit.:
This edit did not correct critical issues with the post - view the revision history to see what should have been changed.
That is often more appropriate than any of the Reject reasons, but sometimes it is not, because the issues with the post that needed to be fixed were not all that critical, but the edit being suggested was completely wrong, and what we want is a way to tell the user what they did wrong.
Fortunately, although you can't tab-complete their usernames there, you can @ edit suggesters in comments on a post their edits were applied to. If you feel it's really necessary to give feedback to an edit suggester, you can use that method, and they will get a notification of a new message and hopefully read it without digging around in their profile. You can also @ the user in chat if they have been there lately and you can tab-complete their username, or if it's really necessary, ask a mod to superping them in chat.