Mostly by Sicilian and Neopolitan Americans whose ancestors were culturally and linguistically distinct from the Genoese Colombo.
Columbus Day was pushed by Italian Americans largely to show the larger North American community that they "played a part" in the American narrative.
As for myself, I would prefer celebrating Cortes, but that would REALLY piss off the dimestore Indians and the white liberals.
Thorvald Ericson!