I feel that it needed a key feature that made it stand out as a different game, but it doesn't have that. As you say, it has things like reworked subclasses, an abundance of new content, etc, but that content appears to fit the same formula.
Look at the marketing for something like Star Wars Battlefront, for instance. The game is iterating on its previous instalment by adding a new singleplayer campaign, it's what people wanted, and it's a big deal.
Destiny 2 is purely iterative, expanding inwards not outwards, and it's difficult to market that. Especially in some cases where the iteration isn't clearly, objectively better. If for instance you used to play 6 player crucible with your friends, the more focused 4 versus 4 offering in Destiny 2 seems worse. Instead of enhancing the gameplay of the large maps and large scale game modes, they removed them, yet obviously there were people that enjoyed that style of game.
But as I say, it's missing a key feature for them to market it with. Something like space combat, or perhaps cool new social hub features, a new focus on vehicles. It's just all over the place, lots of small changes, most are improvements, some are sidesteps, but in total it's difficult for the user to evaluate that and clearly identify that they'd enjoy it more, or less, before playing it. Whereas, if hypothetically, you had a big new feature (e.g. Space Combat) then players can see that and think 'oh yeah, that looks cool, I'd enjoy that).
But looking at the gameplay reveal. The myriad of small changes, the tweaks to how classes work, the new weapons. Will I enjoy that more? How much of a difference will the plethora of small additions make to the my experience? It's not explicitly clear. I think in general it's likely that all of the small changes will have a significant impact on the quality of the experience that people have, but the game lacks a unique selling point when compared to the first game and that makes it difficult to market.
Also the story isn't new enough for me. I saw enough of those three characters in the Taken King DLC and onwards and it just doesn't feel fresh as a result. I think a focus on new characters could have sold the idea of a new story, and that could have helped quite a bit. Look at how Borderlands shifted between 1 and 2 and placed its focus on story, they introduced iconic, new characters to drive the narrative, and it had a huge impact on the games marketing and appeal.