Why does the Torah write "Mishp'chos Bnei Kehas" (and not Mishp'chos ha'Kehasi" - like it did by Gershon, in Pasuk 23)?
Sifsei Chachamim: They were not worthy to have Hei and Yud (of Hashem's name) appended to them, because they sinned and made others sin. 1 This is why Rashi explained here that Kehas encamped close to Degel Re'uven, which was also located in the south. And that explains why Dasan and Aviram and the 250 men, mostly from Reuven, were drawn after Korach 2 and joined him in his rebellion. 3
Malbim: This is because not all of them camped there; Moshe and the Kohanim were in the east.
Note however, that Pasuk 35 says "l'Mishp'chos Merari", and not ha'Merari (EC).
Rashi: Based on the principle 'Oy l'Rasha, Oy li'Shecheino!' (It says ha'Kehasi in verses 27 and 30! The Hei and Yud are omitted only here, where it says where they camped, to hint that the omission is due to drawing Bnei Reuven into the rebellion.