My guess is that they wanted to target the widest audience possible and with the PS5's backward compatibility they got to release it on the PS5 for "free".
(yes, they are missing the Xbox audience, but technically you could play the Xbox 360 version on Xbox One and Series)
They're also missing the huge PC market. I'd honestly be 100% happy if they just released it with an emulator packaged with it, just give me a convenient way to play on PC.
This gives me a little hope that we can at least emulate the PS4 version (using pcsx4). This can be done already with the PS3 version but the requirements are hefty and it's pretty clunky.