One of the "standard" Chanukah questions is why the halachos of Chanukah are not considered in Mishnayos. One of the standard answers, related in the name of the Chasam Sofer, has always bothered me, as it goes something like Rebbe did not include these Halachos because the Chashmona'im had undermined the principle of La Yasur Shevet me'Yehuda u'Mechokek me'Bein Raglav, thus usurping the power that rightfully should have belonged to the House of the Nasi,
I always had difficulty assuming Rebbe was not enough of a Ba'al Mussar to overcome such a seemingly petty antagonism.
I think, however, that the basis of the question is the contrast to Purim, which is discussed extensively in its very own Mesechta. And a recent Daf Yomi Yerushalmi establishes that the question is the other way around: Why did Purim get that attention - i.e., really both Chanukah and Purim should have continued beyond the redaction of Mishnayos to remain relegated to Torah she'b'al Peh. Why did Purim get a Mesechta?
Says the Yerushalmi (Megillah 20b):
שמעון בר בא בשם ר' יוחנן וזכרם לא יסוף מזרעם מיכן שקבעו לה חכמים מסכת
Shimon bar Ba in the name of R' Yochanan: "V'zichram lo yasuf me'za'ram" - from here [we derive the reason] why Chazal designated for it [Purim] a tractate.
Were it not for that Yerushalmi, Purim would have has the same status as Chanukah - which would have been the default status of a Yom Tov d'Rabbanan of not being considered in detail in a Mishnah.
Yishma Chacham v'yosif lekach.