Remember, South america was far more populated than North America. The reason for this was because South America held the most favorable conditions for farming which led to cities which led to civilizations arising like the Aztecs and the Inca. Of course many North American peoples settled into farming like the Hadanasuanee (or Iroqouis) but due to the snow and the seasons these nations could not farm as much as the Southerners leading to less population growth. And because the various European Plagues wiped out the natives in the Americas both North and south dropped dramatically. But due to the southerners common use of farming they were able to adapt to european systems easier andcould thus grow their pops more. Whereas the Northerners wern't used to farming and thus could not grow like the southerenres.

Also the British colonization policy was to send britishers to the New world to set up the colonies there. Due to this the areas were flooded with British folk far outnumbering the natives. This was partly due to the inability to get native workers early on as all the tribes ran somewhere else when the British tried to capture them. There simply wasn't a native society in North America that could be "conquered" because there were no lands or borders in N. America, just tribal claims and tepees(and other moveable houses) that could be moved and changed easily. These many reasons created a british policy of immagration to the new world.

Whereas the land the spainish conquered coupd be conquered. They had kings, nobility, clergy and peasents. When the spainish defeated these kings, the people plegded loyalty to them because the natives were defeated. And because they had built cities and feudalism they couldn't just run away. After the spainish conquered the New world they decided that they only needed a spainsh minority in powr that could use cheap "encomienda's"(or slaves) to run their colonies. This led to the natives outnumbering the natives and because the natives were used to the farming system they could use the surplus food to grow their population.

Also the british had less useful land, like Spainsh America had gold, silver and plantations whereas British America had littleof that until the 1800's. This led to the british colonies being mire based of the people and their producing powere with a heavy influence on trade. Whereas the Spainish realized the most effective way to gain wealth was exploiting slaves and encomienda's to work the fields and the silver mines to make Spain so rich it had a inflation crisis. But the side effect was that societies cannot be built off pure extraction of resources which was a factor in todays poverty in south america.

But when the Former spainsh colonies declared independance(lets go Simon Bolivar) they realized that there was a major gap between the spainsh born, the spainsh New world born and the natives. To fix this issue the instatuited a policy of intermarrige that would join the 3(and more) communities into one. This was remarkably succesfull and united the different races and decreased racism(though it still exists). And on top of that the fact that most of the people were mixed race people were more willing to claim their Indeginous ancestry leading to a larger overall population of native-ish people.