Because Moses want to use violence to save the Israel when he was young, it was not God’s way. So God trained him in the wilderness to be a humble leader.
Why did Moses keep the sheep in the wilderness for 40 years? Why did not God call Moses when he was young and when he was old and old?
God’s timing is perfect. When Moses was young, he sill had a lot of angry in his heart, but that’s not the way of God’s to save His people. After 40 years in wildness, Moses learned to be patient and obedient as shepherd. That’s the time God called him again to lead the people of Israel.
He has made everything beautiful in its time. He has also set eternity in the hearts of men; yet they cannot fathom what God has done from beginning to end.
He needed the time that he gets be humble and depend od God.