The P’nei Yehoshua asks, “Why was it necessary to use pure, sealed oil to light the menorah? We know that if everyone is tomei, it was permissible to use tomei oil.”
Moshe Rabbeinu descended from Mount Sinai after begging Hashem to forgive the Jewish people for the Chet HaEgel on the day after Yom Kippur. Within three days they began building the Mishkon and it was completed in the month of Kislev. Even so, Hashem postponed the celebration until the month of Nissan.
The month of Kislev was unhappy and complained to Hashem. “Why was I left out of the Chanukas HaMishkon and all of the celebrations? It was finished in my month!” Hashem said, “Don’t fret; there will yet be a time when the Yidden will return to the Bais HaMikdash in your month.”
But Kislev was still not happy; he said, “The celebration for the Mishkon was in purity. Why should my celebration be in tumah?” So Hashem made a miracle and a jug of pure, clean, untouched oil was found in the Bais HaMikdash and Kislev was happy.