Rabbi Avi Shafran writes:
…Why, if oil sufficient for one day was discovered in Jerusalem's Holy Temple when the Maccabees reclaimed it from Seleucid control, is Hanukka eight days long?
True, that is how long the candles burned, allowing the priests to prepare new, uncontaminated oil. But was not one of those eight days simply the day for which the found oil sufficed, and thus not itself a miracle-day worthy of commemoration? Suggests Rabbi Feinstein: Seven of Hanukka's days commemorate the miracle that, in the time of the Maccabees, the candelabrum's flames burned without oil. The eighth commemorates the miracle of the fact that oil burns at all.…
Of course, the answer to that famous question, a question codified by Rabbi Yosef Karo almost 500 years ago, is not Rabbi Shafran's answer or Rabbi Feinstein's.
Hanukka is eight days long because the Hasmonean's meant for it to be a make-up holiday for Sukkot. (Part 1; Part 2.) They expected to retake the Temple in time for the Sukkot festivities but failed to do so. So they made and eight day holiday, mirroring the eight days of Sukkot, just over two months after the Sukkot ended. Then, in the following years, that eight day holiday, eventually known as Hanukka, was used to celebrate the Maccabees's military victory. There was no "miracle of oil."
Later, rabbis – who did not participate in the Maccabees's revolt and did not play a significant role in the state, invented the "miracle of oil" as a way to de-Maccabee the holiday.
History is not a haredi strong suit. Indeed, it is that willful ignorance that keeps the haredi world going.
More on Hanukka here.