Up until now, the Nintendo Switch Pro-Controller in the US hadn’t been approved by the Federal Communications Commission (FCC). Their job is to regulate radio, television, wires, satellite and cable communicate within and between states of the US. Without their approval, you won’t see your item/service on sale. An eagle-eyed reddit user (WeirdGuyInTheCorner) spotted the relevant information on the FCC website. Click here and here to view it.
Now that the controller has got the green light from the FCC, hopefully folk in the US will start seeing pre-orders become available.
Thanks Patrick for the tip!