Its one thing to like less restrictions, some make complete sense, others don’t.
At first I was in opposition to a few races, we can guess who. They had a LOT of turmoil involving demons, having warlocks among their own culture that they were against for so long, why would they? However, as some pointed out, we aren’t NPCs, we are our own individuals and I even done this myself and realized, “Yeah, I can see it happening but…” it then you have to think about, “Is it actually plausible?”
What I am getting at next is the plausibility aspect, we have certain races who are outright infused with specific elements, Lightforged being quite much like Demon Hunters, infused with The Light in their case. How can that work as warlocks? Well, if anyone was aware, our playable warlocks use Soul Shards as their source of power, not our own lifeforce, we can safely use Fel/Shadow magic safely. If it were the case, such as Void Elves, a void-infused race, they would have near-impossible ways to become Paladins. Why? One, they were exiled entirely, no connection to the Sunwell. Two, Paladins are light-infused warriors (Quite metaphorically and literally). They. Will. Not. Survive. And third, if they do manage this, they’d likely wrest back to being Blood/High Elves in that way… Paladins would need a void spec to compensate this very lore reason to happen… But Warlocks? They aren’t using their own being as a means for their power, again Soul Shards to circumvent this.
Now, the coverage of Lore, there’s little reason why ANY race couldn’t be but also how would it be terrible to see a Draenei suddenly becoming and using warlock-specific spells? One would assume you were a Eredar at that point. Lightforged even would likely kill you on the spot and ‘forsake’ The Light like that but thankfully there is a quest that will cover it all, which I am happy they are going to do because none of this would make any sense otherwise.
Any races that were personally ruined by The Burning Legion would have been outright opposed and possibly kill you on sight for such a thing because of what the sight of such a individual could mean.