Over on the Stadia reddit page a user called you_are_breathing is reporting that their Stadia pre-order has been cancelled. While the email from Google is not giving much away I think it is safe to assume that the reason the order has been cancelled is due location.
While internet speed is important, being close to the data centre is also key in getting a good service for Stadia. Hawaii does not have data centres close so instead of the users getting a poor service Stadia may have done the pro active step.
As Stadia is a purely online service we would recommend that you do not get the service until your location is officially live. When we find out more on this story we will let you know.
When ordering Stadia it does state that it is not available in Hawaii and Guam.