With many console releases like Assassin's Creed: Valhalla only now embracing 60 FPS on newer systems, I still find it wild that 120 FPS modes are a big selling point of the new Xbox systems and the PS5 for some. For fast-paced esports titles like Rocket League, though, I understand the desire—and for that title, it's only available on Xbox Series X and Xbox Series S. The reason comes down to how much more work it takes to get 120 FPS on PS5.
That's what Rocket League developers Psyonix told our sister site Eurogamer when asked about the discrepancy between the game's feature-set on the new consoles. To add 120 FPS support on Xbox, all it took was a "minor patch," Psyonix says. To get the same feature on PS5, it'd require doing a "full native port."
While PlayStation lacks its own branding of cross-gen functionality a la Xbox's Smart Delivery, it also simply doesn't have the same consistency across versions. That's part of why we've seen issues with save transfer support between PS4 and PS5 titles and sites warning PS5 owners not to download last-gen versions of their games by accident. On the developer side, another cross-gen feature Sony lacks is 120 FPS support for backward compatible games.
Now, this is feasibly something that Sony could fix, but then you have to start asking about how much work a fix would require and the benefit that would come from it. Realistically, there's only a small number of titles like Rocket League where the makers would endeavor to do any amount of work toward 120 FPS support. That frame rate also remains something that only a small fraction of players have compatible screens for. On top of that, Sony might prefer that developers make true native PS5 versions of competitive games (as Fortnite's doing), as that could entail new selling points like enhanced 3D audio and use of the DualSense's haptics.
Psyonix says its main focuses for 2020 were the free-to-play launch and big updates to its tournament system. That doesn't mean a PS5 port with 120 FPS support isn't a possibility in the future, but when you consider how much more impactful going free-to-play and improving tournaments are for the Rocket League community, it makes sense that Psyonix would punt on PS5 while doing the easy patch on Xbox anyway.