Albert Barnes Commentary 1 Samuel 18:17

"And Saul said to David, Behold, my elder daughter Merab, her will I give thee to wife: only be thou valiant for me, and fight Jehovah`s battles. For Saul said, Let not my hand be upon him, but let the hand of the Philistines be upon him." — 1 Samuel 18:17 (ASV)

Saul had not until now fulfilled the promise of which David had heard; nor was it unnatural that Saul should delay doing so, until the shepherd’s boy had risen to a higher rank.