“ And YHWH has driven out from before us all the peoples, even the Amorites who dwelt in the land. Therefore we also will serve YHWH, for He is our God.”
They protested that they were too aware of the help that they had received in establishing their present position in the land to turn away from YHWH. They had witnessed how He had enabled them to drive the Canaanites (Amorites) out from many places. Therefore YHWH was their God and they would serve no one else.