Tesla Motors has two limitations: volume and development.
They cannot produce vehicles in large volume as GM can, so most of their buyers must go through a waiting list, and many buyers don't like that. GM overproduces and has many vehicles on dealer lots. Even at this present date when the first generation has stopped production, you can find many 2015 Chevy Volts at different dealers and prices may be better than last year when it began selling, yet they are just as new.
In development, GM has the nation's largest battery test facility, although they don't manufacturer them. And Gm has several development facilities acrosss the nation, so a new vehicle idea can be developed, tested, and put into production in less than two years. The best modern example is the present 2017 Chevy Bolt EV, and the best past example is the first generation 2011 Chevy Volt, which was presented in 2008 as a 2009 design, then developed and produced in 2010, making the first customer sale in December 2010.
Tesla Motors, on the other hand, has taken five years for the Model S, over four (and reaching five years) for the still-being-tested Model X, so the Model 3 (which doesn't even have a concept prototype yet) will not be on sale until 2018. and if produced will not reach enough sales volume until late 2018. By that time the second generation Bolt EV will be designed, and GM may even surprise the nation by announcing newer BEVs that can kill off the Model 3 sales altogether.