The demon hunters as a class have been demanded in WoW for a long time. There were many people who supported their addition to the game and many who were against - the main reason, that their abilities would copy the ones of the warlock. Long story short, eventually they were added and I will dare to say,that they are one of the most popular classes in WoW at the moment.
Their addition during the Legion fit the expansion theme perfectly. The sole purpose of their creation was the retribution against the Burning Legion. With Sargeras's imprisonment and Illidan staying behind as his jailor, it would seem, that the Illidari have fulfilled their purpose. The Burning Legion is defeated, their main goal - reached. So,what do they do now? For the game purposes,of course they go and join their factions and wage a war, I find difficult they would be even interested to wage.
The Illidari, quite similary to the Knight of the Ebon Blade, are not just representatives of the same class put in the same class hall to reach a common goal. They were like brothers and sisters, they fought side by side the greatest evil they knew. And,suddenly, with Legion's defeat now they start to care about the Horde and the Alliance war? Why would they even care about these factions? Why would they take part in the war,if they are still are being looked at with a suspicion and disgust, while the warlocks and shadow priests have always been a part of both factions and allowed amongst their ranks? Only because they have lost purpose and they had nothing better to do? Because,they suddenly care about the red or blue colour in the foreign faction banner in the name of which they are ready to kill their former allies?
Have the night elf demon hunters been "forgiven" by Tyrande and Malfurion and accepted as equals amongst the ranks of kaldorei? Are the blood elf demon hunters viewed as equals and allowed to even glance at the Sunwell? Makes you wonder why would they care to fight for people,who see them as abominations and misfits amongst their societies.