There's no one forcing Bethesda, or any publisher, to add Pro Support.
Community Manager posted a comment saying We're working on an update for PS4 Pro and Xbox One X. When we have more information, we'll happily update you. Thanks!
https://bethesda.net/community/topic/110378/no-ps4-pro-support-no-buy/5
Maybe they did not want to delay a release or just want to synchronize PS4Pro update along withe XboxOneX release. Details about the patch is unknown.