Because they considered anything short of pure electric was net negative emission wise: [0]

The CO2 content of any given source fuel is well understood. Natural gas is 14.4 grams of carbon per mega-joule and oil is 19.9 grams of carbon per mega-joule. Applying those carbon content levels to the vehicle efficiencies, including as a reference the Honda combusted natural gas and Honda fuel cell natural gas vehicles, the hands down winner is pure electric

[0] - https://www.tesla.com/blog/secret-tesla-motors-master-plan-j...