The Gallery Moderators are volunteers, C_K. They do what they do out of love for this Community, based on many years of skinning experience and standards which are both objective and subjective.
They will, if contacted by pm explain why a 'low' rating was given to a particular wallpaper.
To ask for an "automatic" explanation is asking the Moderator to devote much more of his/her time to this and I haven't heard of other sites doing that for submissions. I don't believe that explanation needs to be "automatic" in every case.
surely some understanding should be given to subscribers and customers who spend a lot of time in producing their work for free and then making various sizes, preparing additional comments and a preview, and uploading all the work in an acceptable form.
While that is true of the majority of accepted submissions, it's not true of all.
It's not unreasonable to request that the submitter pm the Moderator when appropriate, if just to lessen the load.
To be sure however, there are Moderators who do what you suggest for 99% of their rejection notices. In fact, I don't know any Gallery mod who doesn't.
Maybe you need to check your 'spam folder' for a reply you might have missed?