So both south and north America had European colonizers, indigenous natives, and African slaves, but today Latin America is way more mixed than North America. I don't understand why the majority of Latinos are mixed, but the majority of North Americans really aren't. For example, in Brazil the percentage of black people is 8% despite having more slaves than america which is at 13%. But it looks that way because the african DNA was mixed with a bunch of other things and the average Brazilian has about 21% In places like central america the majority of the population is mestizo (mixed native/spanish) Why didn't America get a population like that? Why aren't the majority of americans a mix of British, native, and african like what we saw in latin america?
More can always be said on the topic but I would suggest you check the FAQ; there is a whole section on this topic. Most of the answers there address mainly the mixing of the indigenous and European population. It would be interesting if someone can give additional info on the African part of the question.
I would only like to point out that you should be careful when interpreting those percentages, one has to understand that ethnic categories in the US work different that in Latin America. The one drop rule meant that essentially you were either Black or White, whereas in Latin America many people that would be considered Black in the US would be perceived as Mestizos/Pardos (Mestizo in modern terms does not necessarily mean exclusively European and Native ancestry, it can also include African ancestry). This makes such percentages difficult to compare and interpret.