Because you’re lumping in the unavoidable disease transfer of first contact with intentional conquest and violence. Take away that, which was going to happen whenever any Afro-Eurasian community first interacted with people from the americas, and you get a very comparable situation to many things throughout history.
The genocide didn’t happen solely after the first contact, the massacre of natives lasted centuries. Many nations were wiped out in the XIX century.
And a quote for you
Proponents of the default position emphasize attrition by disease despite other causes equally deadly, if not more so. In doing so they refuse to accept that the colonization of America was genocidal by plan, not simply the tragic fate of populations lacking immunity to disease.
Professor Roxanne Dunbar-Ortiz