Because the single player/co-op requirements don't punish you if you mess up. For example, in the requirement to finish 5 skirmishes without dying for one of the epics, you don't need to finish five in a row without dying, so you can get killed and simply do another one and the ones you did before still count. In that regard, they dont punish PVP-ers who aren't used to the PVE aspect of the game, since you don't lose points for dying. The competitive portions of the epic bounties, on the other hand, do punish you for playing badly... which a lot of people like me who are not fans of PVP will be. While the mission/story aspects let you keep your gathered points regardless of if you die, you lose them in competitive. I've gone through four control missions, and because of my unfamiliarity in playing against people I have a whopping 20 points (using a fusion rifle and a void warlock even).