The Rambam (2 Megilla 18) quotes a Tosefta in Megilla and the Gemara in the first perek, as follows:
All books in the Canon of Prophets and Holy Writings are destined to become naught when the Mashiach comes, other than Megilas Esther. It will stand like the Five Books of the Torah and like the Oral Law of the Torah which never will be nullified. Even though all the memory of suffering will be extirpated, as it says "for the old suffering will be forgotten and hidden away from my eyes," the days of Purim will never come to an end, as it says "and these days of Purim will never pass from among the Jews and their memory will never end among their descendants."
Nothing from the Canon will be nullified, for every Book has a lesson. All the Gemara means is that only the Megilla and the Torah will be publicly read.
The acceptance of the Torah in the days of Moshe Rabbeinu was under duress (Shabbos 88a). They later accepted it willingly in the days of Achashveirosh, in a manner that the holy light that inheres in the Megilla is literally greater and more splendid than that of our holy Torah itself.
A reader in New Hampshire sent in a comment about what aspect of the Megilla involves the Chasam Sofer's light, which belongs here:
Suggestion: reading the megilla with the proper appreciation and simcha regarding the narrative and the nes creates a bond to torah sheba'al peh greater than can be attained from the Torah itself.(note: great unknown's latter line refers to the part of the Netziv that explains that the Torah is symbolized by the sword; Torah She'bichsav by the gloriously bejeweled scabbard, and Torah She'Baal Peh by the sharp and dangerous blade.)
This is, after all, the light of kiy'mu v'kiblu.
Also, on the most basic level, it is the only torah shebiksav that is torah sheba'al peh.
And on that note, I direct your attention to an analogous if not identical concept in the Netziv, in an addendum on Purim at the end of Shemos (after Megillas Esther).
When I read it, I immediately checked if my katana was sufficiently sharp.
Two good things that add to this discussion:
I At a kiddush, we were discussing the difference between Sinai and Purim. One individual put it nicely. He said "There's a big difference between Kabalas Hatorah under a mountain and Kabalas Hatorah on top of a mountain."