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…

All the cities of the children of Aaron, the priests
Those of the Kohathites that came from him: [were] …

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…