Why might Native Americans living today be uncomfortable with Columbus Day as it had been celebrated through most of the 20th century?

They might be uncomfortable because some people might dress up as indians.

It's hard to come to terms with the fact that there's a nationwide celebration in effect for an explorer who has been credited with "discovering" America when, in fact, he did not. The Natives been living here far before these people knew about their land. That's the most uncomfortable part about all of this: this man is being celebrated for discovering a country he didn't actually discover at all. Of course that might make people uncomfortable.