Tim Sweeney has said in a tweet that thee reason that Stadia does not have Fortnite is that the userbase does not justify the port. In tweet asking about a Stadia port, Tim Sweeny indicated that Stadias userbase is not high enough to justify the cost it would bring to the platform.
There’s not a deep reason. We fully support Stadia in Unreal Engine however the effort required to release Fortnite updates weekly in sync across 7+ platform is extreme and that makes it hard to add platforms that don’t yet have mass market user bases.— Tim Sweeney (@TimSweeneyEpic) July 28, 2020
Fortnite is arguably the most influential game at this moment time. With this news this will be a disappointment to many people.