Lol.
It's obvious why Void elf Paladins are not a thing yet.
No, it has nothing to do with "lore"; because Blizzard has no problem retconning things, and furthermore we already have example of Void Paladins serving the Twilight's Hammer.
It simply has to do with the fact that each Paladin has a unique racial mount that would have to be created for every new race that gets Paladin, meanwhile Rogue, Mage, and Priest have nothing unique that has to be designed for every new race.
Since modern Blizzard cuts costs everywhere they can, they're not thrilled to have to make special paladin mounts for Night elves, Worgen, Trolls, Void elves, Nightborne, etc.