This has to do with colonial settlement policies. It's very instructive to compare what happened, broadly, in North and South America during the colonial period: the English were not very effective at converting native populations in North America, and adopted a kind of walled-settlement mentality rather than intermarrying with or attempting to govern native populations. The Spanish, by contrast, took over existing political structures and were much more successful at conversion in South America, developing states in which much of the population was indigenous and in which there was much more intermarriage between indigenous and European people than there was in North America. In short: the English (and, later, the US government) mainly tried to remove indigenous populations: the Spanish tried to convert and rule them. This is REALLY broad-strokes and wincingly general, but it does explain to some extent why more people in South America would say they have indigenous ancestry than people in North America would today.