Because Nissan was a little stupid and didn't include any sort of active cooling system for the battery. No heat pump, no liquid cooling, not even an air-cooled heat sink (unless you count the flat outer surface of the battery). There is a resistance heater, yes, and that helps in cold climates, but it does nothing beneficial in warm climates.

As a result, many (if not most) SoCal and Arizona owners have experienced quite significant battery degradation [0]. That translates into reduced range. Almost every other manufacturer includes some sort of battery thermal management system. That is why, for example, Tesla Roadster batteries average about 85% of initial capacity after 100,000 miles in real-world use[1].

[0] http://www.autoevolution.com/news/arizona-nissan-leaf-batter... [1] http://www.plugincars.com/tesla-roadster-battery-life-study-...