[suburbanite]

I could be wrong, but I feel like Latin America lacks a massive hub, and all of the smaller airports are close enough to Miami to make dedicated routes feasible. Like Medellin, Cali, Cartagena, Barranquilla and Bogota (and that's just Colombia) all have direct flights to Miami, whereas Heathrow acts as the primary North American gateway for Europe and beyond. If everyone in South America had to funnel through Mexico City I'm sure that route would dominate.