I think it's fair to assume that Varian can't be resurrected because his soul blew on the Broken Shore. That's kind of what Fel magic is about, consuming soul for energy. That was the premise behind the Warcraft movie, anyways.
Whereas Vol'jin had a reasonably normal departure to the spirit world, where we know damn well that Bwomsamdi was going to be waiting for him. Hell, I wouldn't be surprised if Bwomsamdi himself was the Loa that urged Vol'jin to crown Sylvanas as Warchief, because as far as Bwomsamdi is concerned, that's the decision that's going to keep the flow of souls to the Shadowlands going. It could be an easy "Thanks for killing thousands for me Sylvanas, here's ya boi Vol'jin back".