I think I wasn't alone when, about ten years ago, I suspected Apple was working on a high-end image manipulation application along the lines of Photoshop. Just in case Adobe pulled the plug on the Mac version. My wild guess: Apple never needed to pull the trigger, so it kept improving it in secret. Apple still doesn't need to release it, but there's no reason not to either. Personally I don't use that kind of software but that won't stop me enjoying the sideshow.
EDIT: Apple will update the Mac Pro but later than customers would like.