My explanation, unfortunately, will be rather lackluster, though I hope enough to satisfy the question. Doubtless someone better read than I am will be able to expand on what I say should they come across this thread.

Britain, and generally the non-spanish colonial powers settled fundamentally different regions from the Spanish. Obviously in climate and environment, but much more importantly in the peoples that occupied said regions.

Britain colonized the eastern coast of what is right now the United States. There, the indigenous peoples they encountered were nomadic, often hunter-gatherer societies, and even where rudimentary agriculture was present, urbanization was absolutely nonexistent. So you've got a sparse population who have no real traditions of agriculture as candidates for slavery. Would they be useful? Surely the native American population on the east coast was neither large enough nor in any way part of an appropriate culture to form the basis of a would be plantation economy. We'll get back to this later.

In the meantime, Spain conquered some of the most advanced societies in the Americas: the interconnected mass of indigenous sedentary tribes owing fealty to and including the Aztecs, and the centralized and cohesive Inca empire. Though of course there is a lot more substance to the conquests of either, that's not the point in the context of choosing slaves. The point, instead, is that the Spanish colonies now brought into the fold large, agricultural societies with sedentary architecture and sophisticated agricultural practices. These natives proved much better candidates as the foundation of a system reliant on agricultural output.

The Spanish at first installed a system resembling European land ownership and serfdom under said landlord, called encomienda, you can see this as their variation of slave plantations. It was a form of reward for the conquerors who had brought the king his new domains, to be inherited as any land held by a noble anywhere else. On these encomienda estates, natives who had previously been part of the conquered communities, worked in conditions and with rights akin to, if not lesser than European serfs who were tied to the land. One Bartolomé de las Casas, a historian and theologist who had originally taken part in Hernán Cortes' conquest of Mexico later fought in the Spanish royal court for the abolishment of the encomienda system, and the emancipation of the natives (and later on that of enslaved blacks as well), citing the absues they endured under the new Spanish landlords. Encomienda survived regardless despite efforts to eliminate it, and eventually died off as a result of intermarriage with the locals and the children produced by these marriages not being bound by the serflike obligation system (also the mass death of natives due to imported disease). Note also that the Spanish did use African slaves, primarily in mining precious metals, though plantations or estates similar to North American plantations maintained a majority of native workers/slaves (though the latter label isn't wholly fitting).

In North America, as I said, the native population was simply too scarce to make (and forgive the inhuman terminology here) a worthwhile investment as slaves. Rather than enslaving and 'breeding' the natives in the north to make up the slave population it was simply more efficient to import African slaves en masse: they provided the needed manpower in a much shorter timespan than enslaving natives would have, and an already semi-sedentary cultural background for the imported meant a much easier time putting said slaves to work in producing the new luxury crops native to the new world.

This early reliance on African slaves (and the culture of slavery-justification specified to their ethnic demographic which developed) then majorly contributed to the white population segregating from the native American population (hence the dominance of non-native and non-mixed ethnicities in the US), as well as a preference for genocide and deportation over integration. A similar pattern can actually be observed in Argentina, where the dominant ethnicity is not a mix of whites and natives, but rather a culture distinctly based in European immigration, with a significant black population to accompany them, the remnants of slavery just as in the US, and a rather minor native presence.