This is such a big deal lately and it is frustrating that it is not a thing. I heard that it might happen for Rocket League, but then Sony said no. Is there a reason why they do not allow for it? Are there any technical restrictions that are getting in the way of cross-platform play between PSN and Xbox Live?
There is absolutely no reason why this can't be done. There are some cases where the owners of the game servers may not want the sharing to be done, but generally speaking there is absolutely nothing that is stopping cross platform play between the PlayStation 4 and Xbox One.
Last gen, Sony were the underdog and wanted to implement cross platform play because it would keep them on par with the Xbox. Why pay for Xbox live to pay with friends when you can get a PS3. Microsoft did not want cross platform play back then.
This gen the roles have been reversed. Sony are ahead and they do not want Microsoft to gain the advantage of being able to connect with gamers on the PlayStation Network because it will mean that someone who owns an Xbox won't buy a PS4 to play with any of their friends.
The only reason we do not have cross platform multiplayer with PlayStation and Xbox is because it will create some parity between the systems and this will hurt whoever is in the lead. I can't see us ever having cross platform multiplayer unless there is some kind of merger between Sony and Microsoft that allows them to share the same online network or something.