Samuel was directed buy God to go to Jessie’s domicile and to review Jessie’s sons, one of them was to be king. Samuel was told not to look at their appearance or stature, it was the heart that the Lord was after.
Thinking through this it seems that the selection of David as king was not for David’s benefit, it was for the Lord’s purpose. We tend to view position personally. That is a position is mine. That does not seem to be the case in the Lord’s economy, a position is for the Lord, for His glory.
The Lord’s standards are different than ours, mine, or the world’s. It is not about education. It is not about leadership traits as taught by the business schools. It is about heart.
Anointing David in the presence of his brothers, set up a similar dynamic as that which Joseph faced.
It may be that the sibling were used to shape David as king.
Regardless, one thing that is clear, God does not follow good leadership theory in choosing the leaders of His people.