Because Magic is so varied, you'd have to create a ton of tags.
This is only a small list of what can be done, magic: the gathering keeps getting closed when it needs a dedicated site because of how vast it is. It would be different, like Dungeons and Dragons, there's only one 5th edition, but there are so many packs and core sets of Magic: The Gathering that alter rules, add rules, change banned cards for standard but unban cards for modern. There's also the online games and so on..
Also, you say there is a dedicated boards and game site with a mtg tag so there is no need for a dedicated site. However, there's an ubuntu tag for Unix but there's a dedicated site for Ubuntu because Ubuntu has so many unique features.. like with MTG, there is so much that can't be covered with one tag, it needs a site.