Purim Before Chanuka

 The טור/שו"ע and other פוסקים start with Hilchos

Pesach & continue in order of the year's calendar,
Shavuos,Rosh Hashana, Yom Kippur Sukkos,
Chanukah & ends with Purim.

The Rambam has Hilchos מגילה (Purim) before Hilchos
Chanukah.

Perhaps we can explain the Rambam's reason 
for placing Purim before Chanukah.   

In none of the other Yamim Tovim does the
Rambam teach us the story behind the Yom Tov.

*The one exception is Hl. Chanukah. The Rambam
not only teaches us the Halachos of Chanukah but
he also adds the story of Chanukah.

In other words, Hilchos Chanukah is also a 
lesson in history. Perhaps, the Rambam wants
to be historically accurate by placing the story
of Chanukah after Hilchos Purim
which happened 200 years before Chanukah.אמבצי


           
*Harav Hagaon R..Y.B. Soloveitchik zt”l (Boston) explains

Why the Rambam felt it was important to teach us the story
& reason we celebrate Chanukah?
 (unlike all the other Yamim Tovim)

The Rambam in his הקדמה writes the reason he called
his Sefer, “משנה תורה” because if you learn תנ"ך
and his Sefer (משנה תורה) you won’t need any other Seforim.

For all the other Yomim Tovim, the Torah gives the reason
why we celebrate it. For Purim, we have the מגילה.
The only Yom Tov not mentioned in תנ"ך is Chanukah.
Therefore. the Rambam need to write the story of Chanukah.