Because bungie hates voidwalker. They want Solar to be the poster child for Warlocks. If it isn't well, it's daybreak. If it isn't some ugly looking bird armor set, it's still tied to a bird. Leading up to void 3.0 and the launch of Void 3.0, everything about warlock was stripped away. Loosing atomic breach melee and soul rip for pocket singularity. Hhsn made useless. Everyone else got to keep their ohks and receive new ones, too. Make all unique grenades universal. Remove chaos accelerants damage boosts while keeping it the same as it has been since practically the launch of d2. Make devour universal hell they made feed the void universal for over a year. There's no true benefit to making a target volatile unless you run a universal fragment, apply weaken via grenade, another universal fragment, we still have two nova bombs when we should have a void scythe being warlocks very first melee based super. Get rid of vortex its gotten 20 buffs at this point, and it's still -blam!- compared to cataclysm. Nova warp is still recovering. The real kicker is that Titans are better at being vampires than voidwalker is. All Voidwalker has going for it is Devour and Blink that's -blam!- it.

Bungie is unlikely to add a 4th super. Also pretty sure next time we get aspects Void Warlocks get one.