Because it's an other version of an official album.
Hmm, allow me to explain. Albums added to band discographies must be "valid" albums. We consider an "album" to be acceptable, or "valid," if it is a
complete and official work which has been released and distributed to the public.
A work is complete if it's presented as the finished version of that album. In other words, it's the intended final form of the album, with a fixed tracklist released together in a shared package. A work is official if it is or was clearly sanctioned or authorized by the band or its agent. In other words, we run on the mentality that in order for something to be official it needs to originate from the band. And, lastly, a work is considered released and distributed if it is or was clearly and openly made available to a public audience physically or as a full digital download (streaming does not count).
We have these guidelines because we want to ensure (a) that what we include is held to consistent standards and (b) that those standards recognize what we consider to be meaningful musical works. We expect valid albums to be complete because we expect the album to be done. We expect valid albums to be official because we expect the album to be something the band wanted to publish. We expect valid albums to be released and distributed publicly because we expect the band intended for the album to be widely available.
Other versions of valid albums are treated differently. We do include bootleg/disputed versions of valid albums because we want people to be aware that the version of an official album that they've come across is authentic or not. We don't extend this to band discographies because it is not in our interests to list unofficial albums. The ONLY exception to this rule is Mayhem's "The Dawn of the Black Hearts," which we've included because it has substantial historical value to the metal scene at-large. You can't say that for 99.9% of the bootlegs out there, let alone 99.9% of the metal albums ever released.
EDIT: One last point that's extremely important. Information added to the site MUST be verifiable. If you don't know for certain about something, and can't back it up with a source if requested, then that information should not be added. We don't tolerate guesswork/assumptions, and anything that is guesswork/assumption would be removed. We understand that it's sometimes impossible to find the truth about something, but it's better to be cautious and keep information off the site than to include something that is guessed and possibly false/wrong.