'Rashi' citing the Beraisa of 32 Midos: We rearrange the verses to say Hashem ha'Tov Yechaper b'Ad Kol Mi she'Levavo Hechin Lidrosh ha'Elokim La'asos Pischo... Af Al Pi she'Lo Asu b'Taharas ha'Kodesh Ki Lo Taharu." All of Yehudah became Tahor, for they were close to Yerushalayim, so they could become Tahor and return to Yerushalayim. Many from Efrayim, Menasheh, Yisachar and Zevulun are not to blame. They were far, and came via letters. They properly did not delay to become Tahor before coming. Since they were not negligent, atone for them!