Well, this is the most straightforward and honest answer Sony has given as to why they block cross-platform play on Fortnite. PS4 players cannot play with Xbox One/Switch players, while Xbox One and Switch can play together no problem. Sony has addressed this before, but now Sony CEO Kenichiro Yoshida has given the most succinct answer yet.
"On cross-platform, our way of thinking is always that PlayStation is the best place to play. Fortnite, I believe, partnered with PlayStation 4 is the best experience for users, that's our belief.”
I mean, other answers from Sony have been more diplomatic, but I guess Yoshida felt like cutting to the chase this time!