Alright, I’ll bite. I’ll explain what the goofy backstory is with all this, as a show of how pointless it is to argue anything that has to do with WoW lore.
Alleria isn’t a void elf. She consumed a void essence to obtain more power, and learned to resist the whispers. Void elves were a small sect of blood elves who decided they wanted to further the research an old enemy was doing (the one you destroy at the end of the Tranquillien questing experience).
Turns out blood elves were right to fear that guy, because the ritual went horribly wrong and the elves only barely got saved, but remained infused with the void.
Exiled, lost, and helpless, Alleria helped them control the whispers because she is the only one so far who hasn’t gone insane from taking in so much void power or whatever. Thus, she’s the only one able to teach the type of control it takes.
Void elves are like a ritual gone wrong, with void marks on them to show what’s within. A science experiment gone wrong, a fluke, a miracle last second save. Alleria is a regular high elf, who successfully completed her own separate ritual with the help of that Locus-Walker dude and some Adventurers (the players).
Basically, Alleria and the void elves both did separate, different science experiments on their own. Alleria knew what she was doing and had a teacher, and so she succeeded. The void elves had almost zero clue, failed, nearly caused a catastrophe and were saved at the last second, but too late to undo the damage to their bodies.
Light-skinned void elves are just a player choice compromise to end the high elf debate because they want to give in but can’t agree internally on whether they should add a 3rd thalassian skeleton allied race or not.
Lore reason is basically “??? idk fill in the blanks with your own guesses” because WoW has always put gameplay before lore. I’m choosing to just think that light-skinned void elves are ones that are learning to master the void better than their peers due to having studied for longer or harder. Or maybe the visiting high and blood elves just study the same type of void that the current “shadow” priests do because they’re already void priests… Not buying the theory that they’re new void elves, though. The void seems to require some kind of absorption into your body to work as a source of power. Except for shadow priests, apparently.
See? It’s a mess. Might as well not argue about whether or not she “deserves” her blonde hair. Blizzard doesn’t take WoW lore too seriously, neither should you. It’s just a fun game where you can be whatever you want.