Halsin has something bigger to take care of at the moment. He has the shadow curse situation to solve that is an unnatural plague and represent what the druids attempt to avoid in the first place he also feel responsable for this so he is attempting to rettify his mistakes. An outsider could bring the druid back on a more tollerant way an outsider would be the perfect choice because he could have the the perspective necessary to solve the issue. Since Halsin was kidnapped nobody named an archdruid. Khaga probably was named by consensus. But usually is an Archdruid that names an Archdruids the decision is never under question as long the new Archdruid keep to mantain balance in a good or a bad way.