Not for thy righteousness, or for the uprightness of thine heart, dost thou go to possess their land: but for the wickedness of these nations the Lord thy God doth drive them out from before thee, and that he may perform the word which the Lord sware unto thy fathers, Abraham, Isaac, and Jacob. (Deuteronomy 9:5)
God had originally allotted Canaan to the Amorites and other tribes Israel found there (Deut. 32:8; Acts 17:26), but they forfeited their right to it by their evil ways. Should Israel imitate the ways of these nations they too would be driven out. This eventually happened (see Ex. 34:24; Deut. 4:38; 11:23; Joshua 23:5, 9).