They probably use RNG so that if someone is a hardcore player at doing strikes and raids and suck at pvp they still have a chance to get something. Vice versa if someone loves pvp but sucks at staying alive doing strikes they still get a chance. It allows players to play both parts of the game so they have a chance at loot rather than people not doing crucible because they suck and don't want to do it because the know they won't get anything