Combination of it being the only Raid (with top level gear) for an entire year, and the fact that it was way too reliant on mechanics. My regular group nicknamed this raid King's Wipe since if you lost one or two teammates in most sections you were better off wiping and starting fresh since it was close to impossible to fight your way back or manage the mechanics without a full team. To me the King's Fall raid is a complete overreaction to Crota's End which was clearly put together too quickly and could easily be solo'd. As such they wanted to ensure that the new raid required a full team to complete and they did that by implementing Boss HP and mechanics such that anything less than a full team would have a very difficult time managing any one section should they lose a player or two in the process. Unfortunately by doing so they robbed players of the experience of being able to claw their way back in clutch situations as they could in VoG which in my opinion still had the best mix of combat and mechanics. WotM has corrected this a bit requiring mechanics which are complex, but also leaving room to lose a couple of team mates along the way and still eek out a victory by persevering, so that not every death requires a complete team wipe.