edit: correction, 3 ali races, not 2.

Because undead can't wield the Light lore-wise. DUH. It literally hurts/kills them if they try.

And lemme guess, you hate Blood Elves because they look "gay", right? Eat shit.

I think that's more of a religious subject for the lore of the game, orcs were into shamanism before the demon-frenzy and seems to be returning to their roots, trolls have been influenced by orcs into the same thing, and before that there were into vodoo magics and stuff, same nature-loving religious views goes for the tauren, and I don't know but the forsaken seems to be released of the Lich King's control but they still retain some evil or anger or a desire for revenge, whatever they call it and one of the canons of the holy light is "bringing happiness to others", so I think they are too far away from the light to become paladins (besides who would trust an undead casting exorcism?)

For some reason, at least unknown to the mortal races, only certain races can learn the abilities of a paladin. Those were dwarves, humans, and then the space goats up in the sky. Not even high elves could be paladins and this often enraged certain high elves. This particular fact really adds to anyone wanting to RP a blood elf paladin. Being able to attain something that was previously unattainable to your race. Theoretically speaking, it may have been the race's arrogance, or perhaps that the intelligence/s (naaru?) behind the light denied them for some reason.