Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Jos 21:1-8) Cities for the Levites.
(v. 9-42) The cities allotted to the Levites.
(Jos 21:43-45) God gave the land and rest to the Israelites, according to his promise.
The Levites waited till the other tribes were provided for, before they preferred their claim to Joshua. They build their claim upon a very good foundation; not their own merits or services, but the Divine precept. The maintenance of ministers is not a thing left merely to the will of the people, that they may let them starve if they please; they which preach the gospel should live by the gospel, and should live comfortably.
By mixing the Levites with the other tribes, they were made to see that the eyes of all Israel were upon them, and therefore it was their concern to walk so that their ministry might not be blamed. Every tribe had its share of Levites' cities. Thus did God graciously provide for keeping up religion among them, and that they might have the word in all parts of the land. Yet, blessed be God, we have the gospel more diffused amongst us.
God promised to give to the seed of Abraham the land of Canaan for a possession, and now they possessed it, and dwelt therein. And the promise of the heavenly Canaan is as sure to all God's spiritual Israel; for it is the promise of Him that cannot lie. There stood not a man before them. The after-prevalence of the Canaanites was the effect of Israel's slothfulness, and the punishment of their sinful inclination to the idolatries and abominations of the heathen whom they harboured and indulged. There failed not aught of any good thing, which the Lord had spoken to the house of Israel. In due season all his promises will be accomplished; then will his people acknowledge that the Lord has exceeded their largest expectations, and made them more than conquerors, and brought them to their desired rest.