For void elves, the void is vampiric and blood elves used to feed on energy in TBC. It's not random at all. It's just people taking a race that looks like a vampire and playing it like a TBC blood elf.
Blood DK's entire toolkit also justifies it. Not random at all.
Let people have fun.
They're not even the only ones in concept and aesthetic.
Literally any DK can be a vampire if they want to.