Why is there no mention of the miracle of the candles burning for eight nights in Al Hanisim?
Somewhat related to Shalom's answer, the Lubavitcher Rebbe zt"l explains (Likkutei Sichos, vol. 25, pp. 235ff) that the miracle of the oil (a spiritual victory) overshadows the miracle of beating the Greek armies (a physical one) - because the latter was itself part of a larger spiritual war: they tried to make us "forget Your Torah and abandon the decrees of Your Will"; ordered the Jews to "write on the horn of an ox that you have no portion in the G-d of Israel"; etc.
So when we want to mention and describe the miracle of the military victory, we have to omit mention of the miracle of the oil. Conversely, when the Gemara explains מאי חנוכה, it mentions the victory only incidentally.
Al HaNisim was designed to be placed in the blessing of "Modim", in which we give thanks
The focus is on the "everyday" miracles that occur within the bounds of what we call nature. So Al HaNisim likewise focuses on the military victory, which wasn't (blatantly) supernatural.
(This isn't my answer; I heard it from someone contemporary whose name I sadly don't recall.)