Well I can tell you it's not simply out of a technological partnership if that's what you're thinking. That might play a role in it but it's definitely not the only one. FWIW Epic's always done their demos on Sony systems btw, going all the way back to PS2.
So really this is just more of the same but it's also convenient in providing ammunition for online console spec debate (and some people who take that too far into warrior mode). We know it'll scale down that's for certain, but it doesn't mean much saying it'll scale down to, say, Series X when there's honestly no figures into what specific assets are being streamed in the demo during some of the segments, how often unique assets are coming in and getting jutted out, if compression range for all the assets is uniformily the same (if you have a texture that never goes beyond a certain LOD you can essentially just store and stream a low-level mip of it. That's one of the "scale down" aspects you might be referring to for other platforms but there's also no telling if that was done for this specific demo), etc.