“as MDG pointed out, if we were to celebrate anything, it should be the end of the war”
For Chanukah we don’t observe the end of the war, which was almost 20 years later.
“then how can anyone use that reasoning for days other then 5 Iyar itself?”
Rav Soloveitchik z’tz’l agreed that IF you are to say Hallel for Y”H then you may ONLY say it on 5 Iyar. My rav seems to be the only one who followed him on that one.