https://www.gamespot.com/articles/ps4-cross-play-restrictions-are-all-about-money-sa/1100-6459901/

The lack of PS4 cross-play in Fortnite and other games can be chalked up to money, according to former Sony developer John Smedley. The ex-boss of Daybreak Game Company, formerly known as Sony Online Entertainment, said that he saw the rationale first-hand when he was at the company.

"[W]hen I was at Sony, the stated reason internally for this was money," he said in a tweet (via Eurogamer). "They didn't like someone buying something on an Xbox and it being used on a Playstation. simple as that. dumb reason, but there it is." This was presented in the context of telling fans to keep up the pressure.