Because it is bad, degrade the horde, make the title once a great responsibility into a joke, after Garrosh we want stability not bullshit internal conflicts again, put this plot now to the alliance.
With sylvanas however, it can't be stability, she start a full war and now is doing everything to win this war, with the pragmatic moves everyone likes, saying that, a undead high elf cannot be warchief in any means, and this mess/plot need to be finished quickly, so we can have stability/faction pride again
they can revive vol'jin and make him warchief again, or thrall, until his son is grow up, this way, virtually we will not get "another" leader