I already replied to this but upon further thought I am pretty sure it is because destiny 2 did away with complete weapon sets from raids. In D1 there was a weapon of every type for every raid, I would list them to prove it but I don't remember some of the names but I guarantee that if you looked at the old D1 database every raid has every type of primary, secondary, and heavy in the game. In D2, raid weapon sets are partial, only having 7 or 8 weapons. Think of garden of salvation with only 2 kinetic weapons and no heavies, but FIVE energy weapons. Name three raids in D2 that drop a machine gun. To keep the loot table consistent with destiny 2 they had to cut a few pieces of loot from what existed in d1. It is ridiculous, and I don't know why they made this design change going into d2, but that is probably why atheons epilogue, praedyths timepiece, and praetorian foil got cut. They were the least popular guns in the pool so at least they chose the right ones to drop.