Because the population density was much lower in pre-Columbian America than in pre-Columbian Mexico[0]. Note how Argentina has a similarly low population density - their current population also has a low level of indigenous ancestry.
There are some other nuances as well, like how the existing social structure was preserved in Mexico. Check out Why Nations Fail[1] for the details.
"Population density"is a nice way to put it but hides fact that the local population was exterminated in the british colonies, and not in the spanish ones. That's why there is so few people of aboriginal descent in the US and not in Latin America.
Population density isn't "a nice way to put it", it's data from before colonization began. Regardless of what happened afterwards, the starting conditions were very different in what is now the US and Mexico.
There are some other nuances as well, like how the existing social structure was preserved in Mexico. Check out Why Nations Fail[1] for the details.
0: http://i.imgur.com/RI3uplJ.jpg
1: https://en.wikipedia.org/wiki/Why_Nations_Fail