Because the trick they do to show all the stuff is to stream data super fast from the storage, and since the PS5 has the best SSD in the market, 2X faster than the Series X one, it should work better right now on PS5 than in Xbox or PC, specially when they are a year and a half away from the complete UE5 release so it may not be totally optimized.
In addition to this, PS5 has more than twice than the market share than MS, so more players and devs may be interested on it than on Series X.
UE5 is a year an a half away from its final release. So they may have a lot to optimize, complete and tweak. Same goes with the console hardware, its OS, API, drivers, etc: it isn't final/wasn't final when they were working on this. Make sure that at UE5 release this demo will be better in different ways and one of them may be its native resolution.