Most likely because it wouldn't sell, particularly in Japan.
No matter the engine, developers are currently weighing the value of dropping cross-gen to consolidate services and developer focus. Despite the install base of old consoles, old console users are not using those old consoles as often, so we have something like a 70/30 sales split of current-gen to past-gen. (This number is according to reports of the Horizon: FW launch sales. These figures are otherwise hard to come by in public reports, but in cancelations of past-gen versions of games originally planned to be cross-gen, they cite "focus" and other resource-management reasons why the old versions were dropped. Sometimes there's even still a Switch version planned, despite the PS4/X1 versions getting dropped.) When there aren't sales on a old platform, it's not worth supporting that platform, whether or not the engine can scale down to work on that hardware after some investment of resources. Not only would sales potentially be bad against investment of resources of development time, but you also have the costs of QA and also all the backend support of continuing to keep the version alive.
In Japan, however, PS5 has been struggling, and while PS4 is still carrying on. PS4 and PS5 sales of RE4 Remake have been close to parity, a little in PS4's favor even. That work is worth investing in for the Japanese game buyers, whereas an Xbox One version would have no territory where it'd be the preferred version.