Vayechi 5778

As Yaakov blesses each of his sons, he makes Yehuda's descendants the rulers of all of Israel. "The scepter shall not depart from Judah" (49:10). The Ramban understands this as saying the kingship will never switch from the Tribe of Yehuda to another tribe (as opposed to never falling, which it did numerous times due to exile). Because of this, the Hasmoneans were punished for what they did after the Chanukah story. Yes, if it wasn't for them, the Torah would have been lost forever. However, they fell to their enemies later on because they established the Hasmonean dynasty instead of restoring the kingship to Yehuda. The Gemara in Baba Basra (3a) notes that Herod was only able to come to power because of their punishment.
The Ramban mentions a second possible reason as to why they were punished. Since they were priests, their sole obligation was to serve Hashem in the Beis Hamikdash, as is stated in Bamidbar 18:7. He then brings a Yerushalmi in Horayos (3:2) that speaks of the prohibition to anoint priests as kings.