Why did God say He would not drive the Canaanites out of the land in a single year?
God said He would not drive them out in one year because the land would become desolate and the beasts of the field would multiply against Israel. The conquest was to be gradual—little by little—until Israel increased and could inherit and maintain the land.
Les explains that God's promise to destroy Israel's enemies and make them turn their backs was certain, but He would not drive them out before Israel in one year. The reason was practical: if it happened all at once, Israel would not be able to keep up with it, the land would become desolate, and the beasts of the field would multiply against them. Therefore God would drive them out little by little until Israel increased and inherited the land. Les also ties this to the promise given to Abraham of broad boundaries from the Red Sea to the Euphrates and warns Israel not to make a covenant with the inhabitants.
I will not drive them out from before thee in one year; lest the land become desolate, and the beast of the field multiply against thee.