I do believe the Leviathan raid lacks fun factor. Then you have piss poor rewards and people lose interest. The lack of checkpoints also hurts. Players are penalized for running the raid over multiple nights or picking up a checkpoint from a friend.

I think both the “revive him or it’s a wipe” and the inability (or extremely hard) to complete with less than 6 even if you started with 6 players means you have to be flawless throughout the whole thing. Not that it isn’t possible but for many people, they don’t have the patience to sit 6 hours to “eventually get it done”. I feel that it’s turning off casual players (the group Bungie tried to cater to).