Why did God tell Abraham that Israel would have to wait 400 years before taking the land?
God told Abraham that Israel would wait 400 years so the Canaanites could finally reach the epitome of their wickedness. Their idolatry and child sacrifice had to become fully developed, allowing God in justice to tell Joshua, "Don't spare a one."
Les explains that the Canaanites became so involved in idolatry that they offered their own little children into the fire as sacrifices to pagan gods. That is why, back in Genesis, God told Abraham the children of Israel would have to wait 400 years: the Canaanites would finally reach the epitome of their wickedness, and then God in justice could tell Joshua, "Don't spare a one." The passage goes on to the smoking furnace and burning lamp passing between the pieces, fulfilling the ancient ceremony of transferring title deed to the land.