one reason may be...
One reason is that Apple and Adobe have to maintain some stability between their products, if you ask any graphic designer or any artist, Photoshop is the greatest program ever invented, then again a filmaker would argue that Final Cut Pro is, I don't know about that, but PS to me is so complete that a app from Apple resembling PS would be redundant and I would still prefer PS.
If you ever looked at the splash screen from Photoshop and see the amount of people who have developed it in it's listing and the amount of patents PS has then you run into a river of possible consternation, why would Apple license that many patents to make an app that would in some way resemble PS or do the same thing that PS has? PS is Adobe's flagship app just like Final Cut pro is Apple's masterpiece. Another reason is that Adobe is not developing an OS of its own, any resources that Apple may have will be mainly utilize to further enhance the OS X cosmos and not to develop its own PS, Adobe actually frees Apple from going into an arena where Adobe is already nicely settled in, just like Adobe is free to develop apps for MS and relieves them from worrying about developing an OS that may take them forever to do and could actually be OS X-like but would it be as good as OS X?