Why are there no American riders left in MotoGP?

https://www.motorsport.com/motogp/news/american-riders-wayne-rainey-interview/4320275/https://www.motorsport.com/motogp/news/american-riders-wayne-rainey-interview/4320275/ https://cdn-4.motorsport.com/images/amp/0qXkxPQ6/s6/nicky-hayden-and-colin-edwards.jpg