It is time for Moses to find a successor before he passes away. Moses doesn't know whom to choose so he asks G-d to help him.
And Moses spoke unto the Lord, saying, “Let the Lord, the God of the spirits of all flesh, set a man over the congregation."
Moses could not choose someone to replace himself.
This shows us one more time Moses' humility. By asking Hashem to find the right leader, he is admitting it is time for a new leadership.
Moses gives up even that privilege because he understands the best for him may not be the best for his people.
In memory of my beloved mother and teacher Miriam Rachel bat Israel Itzchak Z"L