Imo they shouldn't have ever been changed from what D1 offered. As you said they had a rotating loot system and when new DLC came out the some would get a fresh inventory like Vanguard and Factions that were never removed from the game. What you are asking is why the vendors inventory were stripped. Bungie got lazy when creating their loot system in D2 and when Forsaken released they made matters worse removing what little inventory Ikora had and stripped the gunsmith while removing Factions. On top of that, not keeping other vendors relevant in this game like the planet vendors or weak at best effort updating the tower vendors. Imo power engrams shouldn't have been used to try to replace the vendors inventory especially with the absurd amount of duplicates in this game. Tess shouldn't have been used to try to fill in that void of missing loot in the loot pool that also removed some of the vendors inventory, cores shouldn't have had to be used other than ( their intended purpose ) and that was only to masterwork. I could go on, but the point is, Bungie needs to work on creating actual loot for this game and stop looking for ways around their unwillingness to put actual effort into this game because they want to be lazy about it. This is supposed to be a loot based game and because Bungie doesn't want to create that much-needed loot is why most of the vendors in this game are useless. Hell, not even Amanda Holliday has ever been used as here intended purpose since the beginning of Y1 of D1. Currently, there are 36 sets of gear missing from this game and if Bungie was doing their job as they should have been doing this whole time, there are an extra 33 sets that could have been added to this game that would make it 69 sets of gear that is loot that could have been added and some of the unnecessary changes in this game wouldn't have "needed" to take place.