I know this one :D I have a friend who volunteers as a tag wrangler for AO3! This is what my friend said about it:

"It is true that we don't make all media types tags anymore. Part of that is about server strain, part of that is because umbrella tags can get messy pretty quick, part of it is because having an all media types discourages people from tagging what canon they're largely working with and then fans of a specific canon get upset (like if we all just used an all media types [fandom] tag, people specifically looking for fic with [character] characterization from the [adaptation] would be kinda upset). There are pros and cons."