I can't speak for Rocket League, but NeoGAF has a possible reason for Sony not wanting to go into Minecraft. It has to do with how players from multiple consoles/devices interact with one another. You need a third party to make PSN and XBL and Nintendo's thing and iOS and whatever to work with one another. And for chat, etc...
The terms 1st and 3rd party are about perspective. In the case of Minecraft, Microsoft's servers ARE 3rd party from the perspective of Sony or Nintendo.
Microsoft's 3rd party solution is Xbox Live accounts and Azure servers.
From my understanding for Nintendo players to join (Minecraft) realms, they'll have to create a free XBL profile and link their Nintendo profile to it. This will enable them to invite people to their realms who are on different platforms.
I need to go find a link to verify this.
Yes, this is how it already works for people on iOS or Android, so no reason to think it would work differently when Nintendo gets onboard.
Bascially, PS4 users would need to create XBL accounts to get into Minecraft. I don't think Sony would EVER go for that. Rocket League is another story. I have no idea why they wouldn't go with what that dev already has set up.
EDIT: Still does not explain Rocket League: