A couple of reasons:
1. It fits in with the brand: hip, innovative, and next-generation.
2. It launched at roughly the same time iAds did, which featured the leaf, which likely drew a ton of mobile traffic, and since this works on iPad & iPhone, it makes Nissan look fantastic.
3. They don't have to maintain two versions of the site, the studio that they paid to do the coding does.
Big brands like Nissan have a large budget to spend on the latest and greatest, and they do it. The studios that do the development are allowed to try new things, and they do. For Nissan, they get a site that looks fantastic, comes in on budget, and garners responses like this thread. The studio that does the work gets to add a site like Leaf to their portfolio of work, and their designers and developers learn a new skill that can be transferred to other high paying work. A win for everyone really.
Big brands like Nissan have a large budget to spend on the latest and greatest, and they do it. The studios that do the development are allowed to try new things, and they do. For Nissan, they get a site that looks fantastic, comes in on budget, and garners responses like this thread. The studio that does the work gets to add a site like Leaf to their portfolio of work, and their designers and developers learn a new skill that can be transferred to other high paying work. A win for everyone really.