These are user supported communities and Apple product teams do not participate here. Therefore, we cannot expound on "why" questions involving product team rationale for lost functionality in Apple products.
The free Skim PDF viewer can still display Postscript and EPS documents when opened in it, but the ability in macOS 13.1 to view these document types in Finder thumbnails is gone. Once macOS 13.2 becomes available, it is a long shot, but Apple may have restored the QuickLook entry for these type of documents.