I wonder if this is because they're just re-using the older engine which probably wasn't optimised for PS5 yet. I understood why Ryza 2 was digital-only for PS5 because of the free upgrade (which basically meant buying a physical PS5 copy in Japan pointless in a sense).Between this and Blue Reflection 2 being PS4-only on the Sony side of things, it looks like Gust have given up on PS5 for now. Ryza 2 was digital-only for PS5, so I guess they've written the platform off as not cost-effective yet with its current install base.
That said, if they do make a Ryza 3, that will probably be a PS5 & Switch release next time. Japanese devs generally don't bother with new hardware for the first few years anyway (heck I'm still surprised NIS America is bringing the upcoming Falcom titles to PS4 in 2022 & 2023).