Why was Solomon chosen as King instead of one of his brothers?

1 Kings tells us that, when David was elderly and impotent, Adonijah considered himself as heir-apparent and crowned himself, but Nathan the Prophet and Bathsheba approached David: > She said to him, “My lord, you yourself swore to me your servant by the Lord your God: ‘Solomon your son shall be king after me, and he will sit on my throne.’ But now Adonijah has become king, and you, my lord the king, do not know about it. Why would David have chosen Solomon, the younger candidate, as a successor?