This is because Allah Almighty addressed them (the pure wives) with His words: "O wives of the Prophet, you are not like any other of the women. If you fear Allah, then do not be soft in speech." [Surah al-Ahzab: 32]
Then He continued the discourse until He said: "Allah only wishes to remove from you every kind of impurity, O people of the household, and to purify you thoroughly." [Surah al-Ahzab: 33]
And Allah Almighty used the word "ankum" (from you) in the masculine form because the intent was to include others (of the household) along with them (the pure wives) in this ruling. Then, attributing the houses to them, He said: "And remember what is recited in your houses of the verses of Allah and wisdom." [Surah al-Ahzab: 34]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q2861