The non-priestly Kohathites had been diminished by the destruction of Korah and his company Numbers 16. On comparing Numbers 26:57 following with Numbers 3:27 ff, two of the families of the Kohathites seem to have disappeared altogether. Hence, it is not surprising that the rest of the Kohathites were sufficiently accommodated in ten cities.
Related Commentaries of Joshua 21:5
Joshua 21:5
5 And the rest of the children of Kohath had by lot out of the families of the tribe of Ephraim, and out of the tribe of Dan, and out of the half tribe of Manasseh, ten cities.