The people of God have forgotten him. The Israelites look more like the Philistines surrounding them than they do the people Yahweh has instructed them to be. Their faith in Yahweh has been reduced to a pluralism in which they worship many gods, none wholeheartedly; and their national memory of the faithfulness of Yahweh is clouded by their distance from his law.
When the Israelites bring the ark into battle, they treat it as a relic or an idol that might be wielded with their whims for force in war. They presume the power of Yahweh might be demanded by silly formulas of turns and shouts, forgetting Joshua’s steps were those of obedience.
They thus presume the role of Yahweh. It is Yahweh who commands Israel. It is he who sends his people to battle to enact his justice, his mercy, his will on earth. It is Yahweh who wields his people as a tool for his glory, for his kingdom upon the earth. But here we see a people wandered far from their God. They devise their own plots and draw their own battle lines, dragging behind them their token for sure victory. They have forgotten. They have lost the right to bear the ark. They are a people commanded to be marked out– by their worship of Yahweh, by their manner of life– yet have neglected these distinctions. Temple practices are corrupt; the people worship many gods; so Yahweh goes into exile.