Q. Why is Chanukah the only festival without its own tractate of the Mishnah?
A. Some say that it is the sages’ way of showing their disapproval of the Hasmoneans, but nonetheless there are several key Mishnaic references to the festival, e.g. M’gillah 3:6 (re Chanukah Torah readings), Moed Katan 3:9 (funeral procedures on Chanukah), Rosh HaShanah 1:3 (the date of Chanukah) and, Ta’anit 2:10 (fasting on Chanukah). Bava Kamma 6:6 asks what the law is if a camel laden with flax passed by a shop and the shopkeeper’s Chanukah light set fire to the flax.
A more likely explanation for the lack of a specific tractate is that Chanukah is not mentioned in the Bible and therefore there is no written law which could give rise to a structure of oral law on the subject (the Books of Maccabees are not in the Tanach but the Apocrypha).
Why then is there no tractate on Shavu’ot, which is a Torah-based festival? Presumably because tradition regarded it as the completion of Pesach, the two occasions being joined by the Omer.