Yahweh said to Gideon, The people who are with you are too many for me to give the Midianites into their hand, lest Israel vaunt themselves against me, saying, My own hand has saved me.
The text does not enable us to understand God’s motive certainly. Simple obedience is what He required. Before God told Gideon to let the larger group of soldiers go home, He gave him a promise that He would deliver Israel with the 300 remaining warriors. This promise undoubtedly encouraged Gideon’s faith.