It seems that all the eight sons of Jessie were strong, and good-looking. Samuel thought that Eliab, Abinadab or Shammah were ideal king material. They had the looks. But as we know God looks at the heart. It would appear that only David had a heart attuned to God.
We only need to read the psalms to know that Samuel, following God’s leading, got it right. However fallible and sinful David was, he still had a heart towards God. When we read further about the exploits of this family we find that the brothers were unkind and mean towards David when he went to the battlefield. Maybe by this time they were jealous. It can’t be easy being the brother of a future king. but God doesn’t make mistakes.