I suspect the mods need to delete dozens if not hundreds of posts every day, which is why they don't have time to mark any reasons for deletion. For a regular member it's kind of annoying to be only told you broke a rule, without necessarily having a single clue about which rule it might have possibly been. I've been there myself, so I sent a PM to a mod. I was given an explanation, and actually got the post restored, although I told the mod I don't need it restored. This might sound familiar, but I also fell for a troll's tricks. The troll then reported my post, I'd assume, and a too busy mod deleted it. And then, like I said, a not so busy mod restored it because despite getting trolled, I somehow managed to avoid writing anything too careless.
That's life. Don't think about it too much. This is just a manga forum.