Alleria is one of the Alliance's greatest heroes, the de facto leader of a politically interesting faction of Horde traitors, and literally Sylvanas' sister.

She also devoured a darkened Naaru, one of the most powerful Void entities in existence, and could probably solo half the Horde with her eyes closed.

Turalyon is the reason the Alliance even exists. He's the one who convinced the human kings to unite under one banner and became the most important Alliance figure over the course of the Second War and following the death of Anduin Lothar. He's also from Lordaeron, y'know that shithole that Sylvanas willingly made shittier to spite the Alliance basically.

Most powerful Paladin on Azeroth, could probably dust all of the Forsaken with his eyes closed and hands tied behind his back.

And they got sidelined. I'm not really sure why they even bothered bringing them back in Legion if they weren't going to use them in BfA. Nothing about their history had any impact on their roles in Legion, they easily could have been new Draenei characters and they've been completely and utterly wasted so far.

Because they're so powerful Blizzard can't conceivably do anything with them because they would decisively end the faction war in the Alliance's favor. And we can't have the Horde actually lose anything, can we?