The United States has large bodies of water on both sides of it separating it from other world powers. (Similar to Great Britain and the English Channel but on a larger scale) and until 100 years ago the only way to transport people across large bodies of water was by boat which was difficult.

Alongside this countries such as Venezuela and Mexico were inhabited by a different type of people, Inca, Mayan, Aztec, etc.. They were not only more advanced than the Native Americans, they were arguably more unified and the countries that dominated these areas. Also taking into account that after the English colonized the first 13, the revolution was backed by other world powers and then America was born. A war was later fought in 1812 over the territory again but it’s important to note that Spain and Portugal were the most dominant in the Southern Hemisphere of Americas. And they both were struggling with trade at the time and it was further away than North America. The people who colonized South America didn’t rebel like the Americans for sometime and were already heavily destabilized and if you look at the Venezuelan crisis of 1902 (I could have the date wrong) they would become heavily in debt.

While it’s true that there was a lot of resources in SA, like I said before the Spanish and Portuguese dealt with some more opposition and some of the territories were completely forgotten due to their inability to sustain a presence there.

Hope that answers it to some extent<3