Saul’s previous victory drove the Philistines out of Israel’s central highlands (see 1 Samuel 13:1 – 14:52).1 Fighting on the plain would allow the Philistines to make good use of chariots and horses, if they had them with them. To their minds, it was also possibly a place where the Lord’s power was limited (see 1 Kings 20:23–28).2
2 And Saul and the men of Israel were gathered, and encamped in the Valley of Elah, and drew up in line of battle against the Philistines.