In parshas Ki Sisa, the portion of Levi is long in order not to read the story of the eigel (golden calf) by the regular Yisroelim in order not to embarrass them by repeating their transgressions. The Leviim didn't participate in the Eigel so therefore we read it by their portion. (Magen Avraham Orach Chayim siman 449 and Machatzis Hashekel there) However, in Parshas Korach we read the story of Korach who was a Levi, during the portion of Kohen and Levi. Why don't we afford the same courtesy to the Leviim as we give to the rest of Klal Yisroel?
So, I asked a Rov and got a few answers:
- By the golden calf, the party lines were split cleanly. When Moshe Rabeinu called out מי לה׳ אלי, all the Leviim came forward and nobody else did. (Whatever amount of participation there was, that announcement and response shows something) Over here, there were many people from Shevet Reuven involved, so it wasn't a shevet Levi vs. the rest of the nation, therefore it wouldn't help to give a regular person that portion.
- Rashi actually points out (don't have the exact place of Rashi, but I saw it this morning), that most of the 250 rebels were from Shevet Reuven
- Even if all 250 people were Leviim, they were still a very small percentage out of the 20K+ Leviim. So a Levi won't be embarrassed being called up, as he most probably does not descend from the rebels.
- Same answer as Double AA's: Korach's children did Teshuva, so there are no descendants of the rebels that would be embarrassed by being called up to the Torah for that portion.