Perhaps Sony didn't realize how important it was to unite platforms with titles as massive as Fortnite. Perhaps they were flooded with bad press and unhappy Xbox and Switch users' critique (read: rage).
According to Sony's announcement they realized that communities around certain games are big enough that adding crossplay support would also benefit the PS4 users. The view they held previously was that PS4 is simply the best way to experience the game and that is why competing consoles will not be supported in the same way as PC for example.
Doesn't really matter anymore, does it?
Sony says that it will enable cross-platform support for the remaining two platforms first in an open beta that was launched already.
Epic Games on the other hand is working on a fix for people that have had to create duplicate accounts for different consoles.
1) An account-merging feature to combine Battle Royale purchases, coming in November.-- Fortnite (@FortniteGame) 26. syyskuuta 2018
2) Enabling unlinking a console from one Fortnite account, and relinking to another Fortnite account. Coming in a few days.