‘ And Caleb said, “He who smites Kiriath-sepher, and takes it, to him will I give Achsah my daughter to wife”.'
This was a kind of test of suitability. Chief's daughters were given to mighty champions to ensure continual strong leadership. Compare Saul's offer in 1 Samuel 17:25. It is understandable why Saul did not fulfil his promise. When he made it he was expecting a champion not an inexperienced young man.