TLDW:
In short. Sony is being odd with how it wants to handle cross play. Even Nintendo is being open to cross platform play. Sony may be doing this because you need an Xbox Live account to play Minecraft even on the Switch (and thus the PS4). Cross play across platforms is a general good idea anyway. All Rocket league's devs say they need is Sony to agree to do it. Sony has not really given decent reasons behind their stance against it. The only one they have given is that they need to protect the children, but... Nintendo is allowing Cross play.