Unfortunately, this is a private marketplace, meaning Envato can hand-pick which items they accept. Your item can even be completely functional, valuable, and high quality – but if for whatever reason they don’t like it then they will reject it, in which case there’s no reason for them to leave a comment.
Now, typically that’s not the case! In reality, they want to sell as many items as possible, but maintain a high-quality and modern catalog, so if they don’t feel that your item meets their “modern and high-quality” standard, they’ll reject it, again without any comment. In this case, there aren’t any specific issues for them to explain. The item as a whole is the issue, and they can’t find any changes you can make to fix it.
If your item does meet their standards but has some issues, they do soft reject the item and go into detail about what needs to be fixed, sometimes even including screenshots.