Because the Leaf just isn't good enough.
Nissan was quite proud when it launched the all-electric Leaf back in 2010. It had big sales expectations, considering that, at the time, there was nothing else like it on the market. The Tesla Roadster was more of a niche model that was ridiculously expensive. But then it happened: the Model S launched and it exposed the Leaf for what it really was: a slightly larger golf car with a hardtop. And now that BMW i3 is on sale, Nissan now has two major competitors on its hands.
Sure, the Leaf costs less, but Tesla is planning that $30k entry-level model for 2016, something that'll really be a thorn in Nissan's side. So what to do? Study your enemy as well as its customers. According to Green Car Reports, Nissan has been quietly interviewing Tesla owners about their cars. Specifically, Nissan has been asking those owners for feedback regarding driving/charging of their cars in normal use, where the car was charged, how often it is plugged in, how many miles it's driven daily, and if the battery's total range was completely used. Nissan also asked Tesla owners what features they liked most and if any were lacking. Will Nissan build a better EV in the near future? It looks like it will certainly try.