Because the Raid isn't easy accessible. It requires you to have 6 friends on the same console at the same time with the same gaming interest or find random people to play with for several hours in a row or arrange times and communicate/socialize on a certain level. Some people don't enjoy that in general or don't want to be a bother. Is that so hard to get? The game has a casual co-op with Public Events which are granted not really deep but could have been the template for a bigger and more structured event. The closest to that is probably the event at Ishtar Cliffs. I wished Destiny would have been a real open sandbox game with a lot real objectives but the way it's designed is imho the reason it's so addictive. It works sub consciously, probably by design, where you imprint it through repetition. As "different" as the Raid is vs. the rest after running it again and again and again to get the 30 armor set+weapons there's nothing unique about it anymore either. It's all about mechanics...