When Ayah of Tat'heer was revealed Muhammad (pbuh) was informed by Jubra'eel about who constituted Ahlul Bayt; they were to be the Prophet himself, Fatima, Ali, Al‑Hasan, and Al‑Husain. At this time Al‑Hasan and Al‑Husain were young children and their future‑contribution to Islam was known only to Allah. Allah knew the pivotal roll Ali, Fatima, Al‑Hasan, and Al‑Husain would play for Islam, thus He specified them for this honor.
Neither Zainab, nor Umm Kulthoom, the daughters of Ali, were included, nor were any of the wives of the Prophet, (neither Umm Salama, nor A'isha, or others).
The Ayah indicates that Allah Himself has honored Ahlul Bayt so specifically, and He promised to keep them “pure, spotless, and sanctified”. Allah had taken it upon Himself to safeguard Ahlul Bayt, to keep them unblemished, untainted, upright, virtuous, and chaste. This is the source and basis of the Ismah [meaning Allah has safeguarded them from: a) sin, b) religious error, and c) forgetfulness.]
As events unfolded through the ensuing 329 years (following the Hijrah), the contributions Ahlul Bayt made to Islam has, without doubt, verified the very essence of this Ayah and confirmed what Allah promised. Ahlul Bayt sacrificed everything at hand for the sake of teaching Islam in its pristine form. They jeopardized, if not sacrificed their own lives and even the lives of their children for the purpose! Ahlul Bayt stood for the righteous and the upright, and for the ideals of Islam, and because of that they suffered dearly at the hands of tyrants and detractors.
source : ttp://www.islamicbooks.info
