The thirteen priestly cities (see the marginal references) were all in the tribes of Judah, Simeon, and Benjamin. Thus, as Calvin remarks, God so o…

And they gave them the city of Arba, the father of Anak
Which from him was called Kirjatharba, ([Reference Joshua 14…

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 t…