With the number of new tracks AJ reviewers have to deal with on a daily basis there’s simply no time to write a detailed explanation to justify each and every rejection. It would slow the queue down to a snail’s pace.
I do agree however that there should be some kind of system where the reviewer can quickly and easily check a tick box according to the issues with the track. There can be different categories such as composition, mixing, production quality, etc. This would enable those who do experience a rejection to at least get a better idea as to where they need to invest their time and energy to get their skills up to speed.