Q. Why is Chanukah the only festival without its own tractate of the Mishnah?
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.