How come I can't give a reason when I select "Reject and Edit"?
I want to give a reason to the user to help them improve. Usually I end up selecting "Reject", going to the post link and separately editing.
I agree it would be helpful if we could give a reason when choosing Reject and Edit.
That said, 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, Personally 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 or at least see the reject about it so they actually see the feedback. But that feature request was rejected on MSE.
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
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 @ editors of a post in comments on that post. If you feel it's really necessary to give feedback to an edit suggester, you can use that method if you can find a post that their edits have been applied to (this works if someone improved their edit, as well as if it was approved), and they will get a notification of a new message and hopefully read it without digging around in their profile. This doesn't work if the suggestion was rejected, though, as only people who have actually edited the post can be notified.
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.