‘Tell me about it’ sounds… different, somehow. It sounds like you're asking for information about something, but that's not quite the original meaning of the sentence. The ‘es’ in the sentence is not the topic, it's the very thing you want to be told: the answer, the reason, the name, the information. I don't think I did a very good job of explaining it, but I don't know that I can do better.
Would it be right in thinking that essentially it should be viewed in context as if you're talking to a group of people and so you would be asking "You all, please tell it to me" but obviously in English I think we would use just the imperative with the assumption it is being applied to everyone who you are speaking to? If that makes sense