It's probably less to do with the level and more to do with the need to either use the clunky out of game LFG forum or join a player-made clan with sufficient members as to do the raid. I think many of the high end raids in WoW have similarly low (if not lower) completion rates for largely the same reasons. Despite putting two hundred or so hours into Destiny myself (I think, at least; maybe less), I've not done any of the raids precisely because it requires me to take out of game steps to get a team together. I don't want to support that bad design, so I abstain. Maybe they'll fix it in Destiny 2.