ModArchive is an archive. Archives usually have a preserving character, which includes that stuff from the past stays available. YouTube, on the other hand, is not an archive but a user-centered content sharing site. Other module sites (e.g. ModLand) have pretty much the same policy as ModArchive. Just think twice before you post something on the internet, that is a wise idea in general.
It's our community's way of avoiding damage to the tracker scene heritage, and we also generally take care that we don't add utter beginner crap to the archive which people may regret later.