Note: this release group is not to be confused with this one, that I created a few minutes ago because I thought it was missing (since it was not listed on the artist page).
Because it has only promotion (or bootleg) releases in it, so it is deemed not relevant to the standard discography and shown only when specifically requested. See at the bottom: “Showing official release groups by this artist. Show all release groups instead”
Release groups get purged if they’re completely empty. “Emptiness” here also means void of relationships—ie., the single/EP and the RateYourMusic URL relationships here means that the release group is not “proper” empty.