Exodus 23:29

I will not drive them out from before thee in one year
This is observed before hand, lest the Israelites should be discouraged, and fear they should never be rid of them; and it was so ordered in Providence for the following reason:

lest the land become desolate;
there being not a sufficient number of Israelites to replace in their stead, to repeople the land, and to cultivate it; and yet their number was very large, being, when they came out of Egypt, as is generally computed, about two millions and a half, besides the mixed multitude of Egyptians and others, and during their forty years in the wilderness must be greatly increased:

and the beast of the field multiply against thee;
there being so much waste ground for them to prowl about in, they would so increase as to make head against them, and be too many for them; or, however, it would be difficult to keep them under control: the Targum of Jonathan adds,

``when they shall come to eat their carcasses (the carcasses of the Canaanites slain in war), and may hurt thee.''