I don't really know how the emacs project is organized but I would have assumed that prolog-mode was maintained on its own by prolog-emacs enthusiasts, and not the core emacs project. I am hard pressed to consider a complaint about indenting in a specific language mode an "Emacs bug"
The only real requirements for getting something included with Emacs itself are that you assign copyright to the FSF and that the code basically works.
There are well over a million lines of Emacs Lisp code included with Emacs (a million and a half, last time I checked), and most of that is for things that the average user won’t need right away, or won’t ever need. The goal is to make Emacs (and by extension Free Software) comprehensive, not to limit the amount of work that the maintainers might need to do. The work they do is naturally limited by the amount of time that they have in a day, so of course it falls to the rest of us to chip in from time to time, especially with language–specific modes where the maintainers may not use that specific language themselves.
