I believe that there are several reasons why the battle has been downplayed, some of which have been touched on here. The overarching issue is this. For good reason, the chachomim in the post churban and post bar kochba period recognized that the exercise of the national/peoplehood aspects of Judaism’s responsibilities was dangerous, potentially threatening the survival of the Jewish people. So they determined to emphasize the personal, familial, and congregational aspects. That involved a complete shift from the beis hamikdosh to the beis medrash, as the focal point, from the shulchan in the BHaM to the shulchan in the home, from the kehuna to the rabonus, from the meticulous performance of korbonos to the meticulous study of Torah shebe’al pe. (The question of re-emphasis now in a time of Jewish sovereignty in eretz yisroel is a very important one to grapple with)
Thus in framing the story – from the time of the first Seder we’ve always been about making these stories fascinating to our families – that which was relatable to the people had to be more about neiros and less about swords and battles.
A second possibility – ambivalence about the chashmonaim. By the time that the customs of chanukah were being formalized, the chashmonaim had usurped the kingship from the descendants of Dovid Hamelech, begun the process of hellenization, and essentially invited the Romans to become involved against the greek faction of Demetrius. It is not widely appreciated that the war with the Greeks and their Jewish supporters did not stop with the capture of the beis hamikdosh, – the Greeks continued to send armies into Judea, succeeding in killing several of the Macabi brothers in battle, and they occupied the Acra fortress overlooking the Har Habayis on the northwest themselves or through proxies pretty much until the Roman influence began. So the fruits of the “victory” were less clear and harder to demonstrate than the miracle of the lights.
Finally, it has long been my question about Chanukah – How much did the “victory” cost us? HKBH acts in history, and within his cheshbon this was a conflict between the urban and the rural, between the elite and the farmers, between the Yerushalmi kohanim who were hellenized and the outliers who were not, between the coastal cities and the hilltowns. It was a civil war, and like all civil wars, it had long lasting consequences. Because of the pushback by the Greeks, the ascending Romans got a foothold, which became a tyranny. It led to the churban bayis. Because the chashmonaim coopted the monarchy, every rebel movement for 150 years was led by a davidic claimant, including the one which eventually morphed into christianity, and we know how much that cost us, up until the present day.
Division has been our yerusha since the brothers sold Yosef into slavery. Chanukah and more specifically its aftermath seems to be yet another manifestation of that.
I’d love to hear a good reason to celebrate the victory as well as the miracle of the lights. I think its just as important, and the sovereignty of the Jewish people demands it. But history tells a harsh tale, so maybe we’re better off focusing on the neis of the lights until we can be unified AS WELL as sovereign, and bring the era of Moshiach tzidkeinu bimheira biyomeinu.