When the Mishna Berurah brings down the reason why we don’t say Hallel in Purim it says because we are still slaves to Achashvayrosh (I know this is only 1 of the reasons that the Gemara mentions). Why don’t we say the same thing for Chanukah? That we are still slaves to Antioches and should not say Hallel?
Interesting question. I once saw an answer to your question in a sefer called Chemdas Daniel pg. 80. He says that the Rambam Hilchos Chanukah 3-1 when he explains the miracles that led up to Chanukah specifically inserts the fact that after the wars the Chashmonaim took the throne for more than 200 years. Why is this point important for the instituting of the holiday? He answers that the Rambam specifically inserted this point in order to answer your question. The reason why they instituted Hallel was because at that point they were no longer the subjects of Greece, but they had their own king.