Best explains why the maya built cisterns

The answer is - droughts.

The region in which the Maya civilization existed, even though in general had a stable climate throughout the year, occasionally, on every few years, was experiencing a long drought. The droughts in this period of time could have been fatal because they were destroying much of the plants that were providing food for the people, and also lack of freshwater. So the Maya came out with a smart method, they started to build cisterns, and when there was rain the cisterns were filled with water that can be used later on when there's crises for it.