The following is from a sefer I have called “Midrashei HaMoadim” a collection of Midrashim on numerous Yomim Tovim
Why do we recite Hallel on Chanukah?
Since it says in Tehlllim (and we recite in Hallel) ?? ?’ ???? ???
For Hashem, He has illuminated for us, in reference to our being saved.
So why then don’t we say Hallel on Purim? After all, the salvation of Purim was both a spiritual and physical salvation.
So why Hallel on Chanukah and not Purim?
We only say Hallel when there is the downfall of the nation who is subjugating us. We say it on Pesach due to the downfall of Paroh and Mitzraim, We say it on Chanukah due to the downfall of the Greeks. After Pesach and Chanukah, we are finally able to be avodim to HKBH, and not to some enemy king.
Purim, however, despite the amazing salvation which we were granted, we were still in galus under the rule of King Achashverosh
(?????? ???? ?)