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].
8 years happens to be the length of the warranty on 2011/2012 Leaf batteries. Either way, combined with the optional $100/mo protection plan that gives you a new battery if yours ever loses 30% capacity, I doubt any Leaf buyers will be on the market before 2021 for any reason other than wanting a new car.
