Read your grimoir kids. The surface of Venus is actually much hotter than Mercury in real life, due to Venus's thick atmosphere and the runaway greenhouse effect it causes. Both were made habitable for humans by The Traveler. The Vex took Mercury by force and turned it into a machine planet. They attempted to do the same to Venus, but were thwarted. You can play on the mechanized version of Mercury on certain Crucible playlists.