War.
Maybe the valleys were already occupied by indigenous people, or perhaps any leaders/rulers who arrive first usurped them, and rather than fight over them, the Maya commoners chose to terrace and irrigate which they knew how to do well. One comment was that a word for house was common to the Indians living there now and to the Maya. That word seems to be similar to a word used by the Seminole Indians which I think is chikee (sp?).