This one is easy to answer...
The Loa are extremely self serving, she was the right choice for the Loa, not for the rest of us...
In particular, one of the Loa Vol'jin worships and has contact with frequently, Bwonsamdi, the Loa of death, who was specifically the Loa who gave Vol'jin his Shadow hunter abilities, wanted Vol'jin to create as much war and strife as he could for no other reason than to send the souls of those who died in that conflict to Bwonsamdi so that he could become more powerful... I wouldn't be surprised in the slightest if Bwonsamdi was the one that recommended, or more likely demanded (since in death Vol'jin's soul would belong to Bwonsamdi, and defying him on death's door would not go well for Vol'jin) her being Warchief.
The Loa are not good and benevolent beings, they are selfish assholes with little to no regard for the lives of mortals.