Along with the announcement that Fortnite was coming to the Switch, people quickly discovered that if you linked your PSN account to your Epic Games account that you will not be able to use it on that version of the game.
Okay, so just unlink your PSN account? Nope, you're still locked out...
I honestly did not even know this was a thing till cageymaru pointed me to it, but it seems as though Microsoft has been talking at Sony a long time about getting cross-play enabled with XBone and PS4. Quite frankly, I doubt this would ever happen as Sony does not seem like the type of company...