Essentially, the new version's inclusion was initially blocked due to the dependency on
mozjs, which was considered by Marc Deslauriers to be too insecure and unsupported to be supported by Canonical (as polkit is a core security component, it and its dependencies would have to be in
main, to be officially supported by Canonical).
There's been some recent activity as
mozjs is now in
main (some versions, anyway) due to being a dependency of GNOME Shell. Robert Ancell was gathering opinions in this Ubuntu Discourse post in January 2020, so you can comment there, if you have use cases for the newer version.