I have two answers;
One is the Ri M’Luniel in Shabbos who says that we have eight days in order to show that it is more important than any other Yomtov, so we have it as long as the longest Yomtov – Sukkos + Shmini Atzeres, which is eight days. Look inside to see why Chanuka is more important than any other YomTov!
My own answer is that if we want to commemorate the highest person in the world who is – lets say – ten feet tall. Would we make a four foot statue for him because everyone else is six feet and only four feet of his height is remarkable?? So too, the oil burnt eight days instead of one. So we commemorate the eight-day miracle with eight days, no?