Why did God cast the Canaanites out of the land?

Why did God send the Israelites into the land of Canaan? Couldn't they have gone to somewhere unoccupied?

The Lord told Abraham in his covenant that the He would judge the Canaanite people for their sin in a day to come:

Gen. 15:13 God said to Abram, “Know for certain that your descendants will be strangers in a land that is not theirs, where they will be enslaved and oppressed four hundred years.
Gen. 15:14 “But I will also judge the nation whom they will serve, and afterward they will come out with many possessions.
Gen. 15:15 “As for you, you shall go to your fathers in peace; you will be buried at a good old age.
Gen. 15:16 “Then in the fourth generation they will return here, for the iniquity of the Amorite is not yet complete.”
Gen. 15:17 It came about when the sun had set, that it was very dark, and behold, there appeared a smoking oven and a flaming torch which passed between these pieces.
Gen. 15:18     On that day the Lord made a covenant with Abram, saying,
   “To your descendants I have given this land,
   From the river of Egypt as far as the great river, the river Euphrates:
Gen. 15:19 the Kenite and the Kenizzite and the Kadmonite
Gen. 15:20 and the Hittite and the Perizzite and the Rephaim
Gen. 15:21 and the Amorite and the Canaanite and the Girgashite and the Jebusite.”

Notice in v.16 the Lord mentions the iniquity (or sin) of the Amorite. The word Amorite is another name for the Canaanite people who occupied the Promised Land before Israel. The Lord allowed the Canaanite people to occupy the land for a time, but then at a point in history, the Lord judged these people by sending Israel into the land to destroy the Canaanites.

The Lord was just to do this since the Canaanite people were wicked in all respects. They practiced child sacrifice and many other forms of immorality. The Lord allowed these wicked people to inhabit the land for many generations before sending Israel to destroy them. Notice in vs.19-21 the Lord specifically said the Canaanite people would be given into Israel’s hand.

Why didn’t the Lord send Israel to an unoccupied land instead? The Lord extended grace to Israel by placing the Canaanite peoples in the land for the years before Israel entered, so that the Canaanites would prepare and care for the land prior to Israel’s arrival. When Israel entered Canaan, the land wasn’t wild and overrun. Instead, the fields were cultivated and the wild animals had been driven away from the land by the Canaanites. In that way, when Israel entered the land under Joshua they found a place prepared for them by God’s enemies. This is mercy and grace for God’s people!

The Lord is good and just to act in this way for the sake of His people. He has rightly judged a wicked culture, while at the same time used them to accomplish a good purpose for God’s nation. The land Israel received was marked out for them long before the Canaanites entered the land, yet God allowed the Canaanites to possess the land temporarily for a time, out of mercy toward them (despite their wickedness) and as grace for Israel.