Not sure about the whole level 500 thing, but I figured they included more exotic quests in TTK and RoI because of the general distaste for RNG systems of reward. We had the sword quests, touch of malice, outbreak prime, and more, each were challenging in their own ways too. Going into D2, I hope they carry on this trend of earning specific items through challenging quests. On the other hand, the average Destiny player consumes content at a much faster rate than bungie could provide it (according to bungie), so it makes sense they kept the RNG system throughout destiny 1 as they couldn't make content that kept up with the community.