Because of the huge divide between where PvE and PvP stands. Destiny was originally more geared towards PvE and cooperative play while the Crucible wasn't focused on too heavily compared to the latter. This is also where the Year 1-Year2 problem begins to show. A helluva lot of content focused on PvE was left behind in Year 1 and was made redundant. Chances are that after TTK dropped no one really bothered with the Archon Priest Strike because there simply was no motivation to run it other than for the replay value, but when they brought it back as a "revamped Taken version" I bet more people jumped on that. The way I'm beginning to see it is that the Destiny devs are trying to put more focus towards the PvP aspect of the game, but because initially they probably didn't really put as much time and effort towards it is why it's pretty crappy and poor-quality right now. Destiny's PvP simply isn't designed for getting the best out of competitive play, at least not in its current state. I do not expect to see any improvement until Destiny 2 after being around for a year to see how's it gotten to where it is.