Weapon balancing.
Let's be real, every time they updated the game, added dlc, or raised the light level, many weapons lost value. Just think of how many year 1 level 30 capped weapons were dismantled! (I stubbornly still have my year1 found verdict and longbow synthesis in my vault). If they carried over Destiny 1 weapons, they would become worthless by the end of D2 year 1 anyways.
But to my main point: Weapon Balancing
Destiny clearly has not figured out weapon balancing if they are still doing it 4 years later. And although there are some things about the most recent weapon balancing that are "okay", for the most part it was pretty bad. I mean basically any special weapon that isn't a siderarm is useless in trials, hand-cannons have crazy range again, blink is ruined, etc. Also, how have they still not nerfed Hammer of Sol?
I think one of the reasons why it was so difficult for them to figure out weapon balancing was because every 4-6 months they added a new slew of weapons, disrupting the previous balance of weapons in the crucible/PvE. Destiny 2 should have a smaller quantity of weapons/armor and instead make sure that EVERY weapon and piece of armor retains its value and usability with every DLC/light level increase.
I for one am glad that previous weapons and armor are not carrying over because Bungie gets a clean slate to make Destiny 2 better than Destiny 1, rather than another repeat of the same game, like all of the DLCs were.