From this map that represents the climate zones and the territories of ancient people in Mezoamerica we can conclude that the Aztecs were living in the higher cooler parts while the Olmecs in the warmer lowlands.
The Aztecs established their empire on the plateau in the the central part of the region where the cool tropical climate is dominant. Also, their expansion was mostly on the plateau, with only very small fraction of the empire being in the lowlands.
The Olmecs on the other hand were occupying strictly the coastal lowlands, thus living and developing in the warm tropical lowlands. Now their empire was significantly smaller than the Aztec one, so maybe if they started expanding they would have gone to the highlands as well, but something like that didn't happened.
