The following is the suggested explanation of Professor David Berger as to why the miracle is not mentioned in either Book of Maccabees or Al Hanissim. (Here is a link to the article on Hirhurim but I think it originally appeared elsewhere, though I can't find it at the moment.)
This question has disturbed many religious Jews. In 1969, a student at Yeshiva University asked me whether the miracle was attested outside of the famous Talmudic account, and I replied that it was not. At the time, I did not have a satisfying explanation for this, and one individual took my response as a denial that the miracle occurred. This was not my intention, but this episode along with questions over the years from other Jews perplexed by the problem led me to struggle with it more than might otherwise have been the case. I now believe that I can propose an explanation that is absolutely convincing with respect to I Maccabees and reasonably satisfying with respect to II Maccabees.
A perusal of II Maccabees demonstrates that miracle stories regarding the Hasmonean revolt and the Temple circulated widely. It is virtually beyond question that the author of I Maccabees heard such accounts, and yet he records none at all. This means either that he did not believe them or that he excluded them as a matter of policy. In either case, the absence of a reference to the cruse of oil--which is troubling only because of the inference that the author never heard the story--poses no challenge to one who believes the account of the miracle on the authority of Hazal. Given the author's consistent historiographic approach, we can be almost certain that he would not have recorded this miracle even if he knew about it.
In the case of II Maccabees, the argument proceeds not from the absence of miracles but from their prominence. Here the author presents various miracle stories so public and so impressive (including, for example, the public appearance of angels) that the miracle of the cruse of oil, which was witnessed by relatively few observers, pales into near insignificance, and he may well have chosen to omit it along with other "minor" miracles. II Maccabees is an abridgment of a five-part work by Jason of Cyrene which has been lost. The full work almost certainly contained miracle stories that were omitted from the abridgment. To us, the story of the oil looms very large. To Jason--or to the man who abridged his work--it may have seemed trivial, particularly since he had an alternate explanation for the decision to celebrate for eight days.
In sum, there are plausible grounds to argue that the authors of both I and II Maccabees could have known the story and nonetheless omitted it from their histories. The absence of a reference in Al ha-Nissim, which is a thanksgiving prayer, need not trouble anyone. The miracle of victory requires thanksgiving; the miracle of the oil does not, and it is appropriately omitted.
See also the many sources collected by Professor Marc Shapiro in this article on The Seforim Blog (some of which argue that the miracle of the oil is in fact ahistorical), and this article by Devir Kahan on Daf Aleph.