Some thoughts on that, not proven by anything that I know of:

People who desperately want to be a leader are rarely suited for it. They tend to hunger for power, rather than feel the need to help the group. It's possible they are competent, but more likely that they will use the group to suit their own ends, rather than working for the group.

People get jealous easily. If someone asks to be leader and gets it, others may feel jealous. If the leader is chosen randomly, it's harder to be jealous, since it was obviously fairly done.