It's a combination of:
1. Mundane loot (like in these types of games the loot you get needs to be priority 1, this is the MAIN thing that drives people to play these games. If you have boring and tiresome loot people will not stick around for it).
2. Boring world - The actual design of the world is fine in terms of layout/mechanics, but it is just not a very pleasing or unique environment to explore. The world of the first game was, from a visual standpoint, more unique and things with it's wintery landscape and design. The world in The Division 2 just feels fairly mundane and a lot of similar looking areas. They need more variety, heck I'd love to have seen weather/season changes which would have helped this out a lot.
3. Repetitive mission design. A ton of the missions boil down to, "go to x place, wait for enemies to attack, fight off wave after wave, hit new checkpoint, rinse and repeat" to the end.