He is be celebrated because he discovered it and brought the knowledge back to Europe which in turn is what lead almost everyone over here. Without him who knows it would have been discovered and people moved over. Yes I know others had found it first but it did not lead to people coming over from Europe.
Yeah, that's a pretty thin reason to celebrate him in the U.S. Literally the only thing most schools teach about him is " In 1492, he sailed the ocean blue". Considering all the he did, its not a lot to ask to keep the national holiday but change who it honors (i.e. indigenous people's day)