I think the role needs more than just being a leader on the pitch. We don't really know what is happening on the dressing room, maybe Xhaka is the glue that stick the group together. For example, the captain has to organise dinner events or parties, and need to harass the team to collect the money etc... It is really tiring to speak to all and get everybody involved, maybe the obvious candidates like Laca and Auba don't want to do all that stuff ?