The Pixel is on a more stock Android software.
Samsung has One UI software on top that needs internal testing before general Beta program can open.
This will obviously take longer.
I too have the Android 15 Beta on my Pixel 7 Pro. Don't forget that as Android is open source software. Google will optimise it for the Pixel phones. So comes out on those first.
Not sure if Google puts restrictions on software release dates as well
But as
@Piper123 has said. When it's available, and your device is eligible to participate. You will see a banner in the notices section in the Members App