I think a lot of people went into Destiny thinking it was going to be multi-platform Halo and when they got Diablo meets FPS with a far weaker story than the Halo series they checked out. A lot of people that bought the first game only finished the campaign and never bought the expansions. So now with less hype for a new franchise and a lot of Halo fans checking out we have a still pretty damn big group of people buying the sequel since it has it's own identity now. We'll see a ton of sales when it really hits PC too.
I'm taking a wait and see approach after I literally spent more time finding a raid, than actually playing one in D1. Without easy access to that content, I realized that long term, the game's value tanked.