Mainly because they had a pretty large choke effect on the entire primary category in PvE. Once we had access to the elemental primaries in VoG, almost nobody used anything but those weapons, which in turn took a dump on the entire exotic spread in that slot for PvE activities.
Morever, they simply aren't needed. Weapons in hard mode are going to roll with naturally higher Attack values than anything in the game, so they already will have an advantage in that category. On top of that, the reward for being able to acquire them from completing hard mode will be that you can in turn bring up any of the other weapons in your collection to their level if you don't want to use the ones you received. If they roll with elemental, this goes out the window since what you'd be infusing them into won't have the elemental.
The weapon slot works better without them, especially with the infusion system in place. I'd rather see the entire current weapon spread retain viability in both sides of the game instead of just PvP. The only one we've seen so far is an exotic, which isn't as bad because it has a higher opportunity cost to access. If you could get an arc primary in that slot without needing to occupy your exotic weapon, it becomes much less valuable. I'm hoping that is a sign they are moving in that direction.
I'm pretty sure 310 is the cap, and normal mode King's Fall (as well as any Exotic Engram decrypted once you're 300+) already hits that ceiling. It'll almost certainly be the same guns with the same attack level on Heroic.
I'm surprised to see you think they made that big a difference. People kept saying Kinetic primaries were useless but for the life of me I've never understood why.
In Vault of Glass the *only* shields are Void Minotaurs, and most people used FB/VoC anyways. Element played no part in this decision; they'd have been 100% identical in performance as kinetics. Likewise in Crota's End there's zero shields other than some ignorable Wizards which you use supers/special/heavy on anyways.
I think the only moment where an element made any sort of discernible impact was Nightfalls. These are 3x a week, and now the burn is only 2x damage instead of 3x so the penalty for not using a proper element primary is even lower. But even then, having something be buffed during a super limited instance doesn't seem game breaking.
Fatebringer was what it was because it was a 13 round mid impact Hand Cannon with Outlaw and Firefly. Vision of Confluence was a high mag, high stability, long range, full-auto mid impact Scout. Until HoW, no other Scout had full auto; until TTK, no other HC had Firefly.
These guns were as popular as they were by virtue of being ideal archetypes with basically perfect perk rolls. The element was just a bonus.
Before the HC murder of 2.0, I'd probably still think that a Outlaw+Firefly Hand Cannon at whatever the new attack cap is would be the absolute god tier primary because of how crazy Firefly is on a HC - the AOE from a HC is enough to actually kill nearby trash mobs. Scouts don't hit hard enough for the Firefly damage to do this kind of damage.
People just got extremely used to these weapons during launch, and since they were basically perfect rolls nothing that came after really took their place. With HoW re rolling you could "make" your own VoC but it'd probably have less ammo and like why the hell bother if you already had the real thing?
Elemental primaries just don't seem too significant either way, but I think they're a cool reward for endgame activities which should stay.