Because tropical (lateritic) soils are not "reusable" in the same way as good thick American topsoil (even though that's a resource that is slowly being depleted). So the slash-and-burn pattern of use continues to eat into the rain forest, because once a plot has been used for a crop or two, the soil nutrients are exhausted, and more forest needs to be cleared. The rain forest itself can be utilized for marketable and profitable products, so that would be a sustainable form of agriculture rather than slash-and-burn, which isn't.
lateritic soils: Soils of humid tropical or equatorial zones characterized by a deep weathered layer from which silica has been leached, a lack of humus, and an accumulation or layer of aluminium and iron sesquioxides. The reddish colour of these soils is imparted by the iron compounds.