Because it goes against the lore. STAHP IT ALREADY. Priest can use Shadow but if purely a Holy priest used shadow they stop using the Light. The relation is that they are polar opposite. So stop it already JUST STOP!!!!!!Okay imagine this then, first off, we don't know everything about the light, that is a fact. We know it is and that it will respond to our call if we believe enough in it, and that's about it, right?
We also know that the Naaru are creatures that are very attuned to the light and possibly has a far bigger understanding to it's nature than what we have.
We also know that when a Naaru get's towards the end of it's life force it transforms into a void creature like the one M'uru is(Although M'urus transformation was forced due the the efforts of the blood elves).
We also know that eventually after being in this state naturally they will transform back into the Naaru they were from the start after absorbing enough souls.
As Krekko stated, darkened Naaru is shadow to light, meaning they are opposites. But the Naaru have a deeper understanding of what this actually means and they could teach us(the player characters) in the quest line how this works, in the end giving us full understanding of how the light meets the dark(?) and what makes them both the same.
Also, a short story about where shadow priests came from:
"Natalie Seline was a bishop from Lordaeron that, after the First War, began to delve into workings of the orc necrolytes who had poured through the Dark Portal and desecrated their hallowed dead. While at first this was done in order to provide Azeroth with the knowledge it needed to fight against such dark magic, her studies over the years became increasingly obsessed with finding out why such dark energies existed at all, and why she began to see the ebb and flow of such energies in the world around her. She began preaching about the necessity of balance between dark and light, but after her death, her books were locked away in the Kirin Tor's vaults in Dalaran
The Cult of the Forgotten Shadow was created through the (re)discovery of her teachings by Forsaken priests in Lordaeron after the capture and refurbishing of the Undercity."
Quote from the Naaru D'ore in game "Without the void, the Light cannot exist." hinting at a clear relation between the two (nothing new of course)
Why would it be impossible for paladins to wield the shadow if priests can do it? They only need to learn how to use it. Also, the light can be used to heal but also harm, so why wouldn't the shadow be able to harm but also heal, if they are complete opposites?