I think it is all because of money. They don't want to lose revenue by allowing players to cross-play. Right now if you want to play with your PS4 friends, guess what, you have to go buy a PS4. That is constant revenue for Sony. Although, I think with all the new games coming out; Fortnite, PUBG, they really should start switching over. If we were still in early 2000's I could understand, but we are in 2018 now. Majority of people love cross-play due to the fact you can remain on your preferred system, but also play with all your friends. All in all, I hope they eventually switch to cross-play. I have a lot of friends on PS4 I would love to play along side.