Others said it here but it's really the features. Steam Link works perfectly to your TV or tablet. They support nearly every gamepad you may want to use, I use PS4 gamepads and never had an issue. They also had the early mover advantage. I don't really see a good alternative other than perhaps the cloud gaming, and that's if you prefer a subscription and don't have a gaming machine. Which has its appeal in the age of not being able to get hardware at a fair price.

If I were in the market for a PC gaming rig today, I'd either use Xbox Cloud Gaming or buy a gaming laptop. As someone that always maintained a desktop gaming PC for 40 years.