Because you are playing in the game as it is after the first "expansion": The Dark Below. When Destiny was released back in September the Vault of Glass was THE raid. Vendor gear went to level 28 (29 with one exotic piece) and the VoG raid gear went to level 30. So VoG raid armor was the only way to play the VoG hard raid....and reach the intial level cap of 30. But when The Dark Below as released, Bungie increased the level cap to level 32...and raised the Nightfall and Weekly Heroic strikes from level 28 to level 30...and raised the highest strikelist strikes from level 24 to level 26. So Vendor gear went to level 31 when maxed, and raid armor you get from doing the Crota's End raid is level 32. Do not play the VoG looking for armor. Only for the raid weapons (which are still some of the best in the game, despite being "only" attack rating 300), and an additional opportunity to get exotic weapons and the Mythoclast. Don't complain because the vendor gear you are wearing has the added benefit of making the VoG hard raid, signficantly easier. People who complain about how hard Crota's end is on hard, have forgotten how difficult it was trying to do the VoG on hard as level 29s.