Why did the LORD tell Gideon he had too many men to fight Midian?

Answer: So Israel could not boast

God said Israel might glorify themselves over Him, saying, "My own hand has saved me" (KJV: "vaunt themselves against me").

“Then the LORD said to Gideon, “You have too many men for Me to deliver Midian into their hands, lest Israel glorify themselves over Me, saying, ‘My own hand has saved me.’”— Judges 7:2 (BSB)