In the US Tesla are the waste majority of EV, and those that are not Tesla usually can't charge anywhere close to fast enough for it to be reasonable to charge at a Supercharger.
The markets are just totally different. In the EU there are lots of cars that could use the Superchargers so you can make way more money there then in the US.
They always said eventually they would open it up to everybody. Its a balance between it being a selling point and making money from higher utilization.