Tesla is a relatively new company. What's hard for them right now is scaling up production. That shouldn't be hard for the existing car companies out there. Additionally, Tesla have made their battery technology open source in order to drive competition and allow (read: motivate) other car manufacturers to build EVs. So basically, Tesla have already laid all the groundwork. The fact that all the other car manufacturers haven't brought something to market yet shows that they are dragging their feet for some reason or another.