From what I understand it has to do with the Microsoft/PS Store". Since you can purchase items for the game through their stores, they don’t want to mix ecosystems. For instance, they don’t want you buying something on discount for the game from the Steam store on PC and then porting those purchased goods over and playing with them on PS4/Xbone.
Sucks, I know. But that’s how I understand it and like said before, it’s unlikely to change. The only way I could ever see it changing is if they remove all purchasable items from the game, which will never happen (as there won’t be a game if they did that, hah).