Probably because it is the only raid where everyone on the fireteam has to work together, especially on Oryx, all it takes is for one person to mess up and the whole team wipes. Combine that with LFG and you have your answer. Personally, it is my favourite raid but this is only true when there is a good team, if not you can spend the whole day trying to grind out a win on a raid that can be completed in under an hour