Why aren't all Mac applications easily portable to Linux?

"OS X is actually (mostly) the proprietary graphical shell on top of BSD. To create an OS X GUI application, one has to follow the API which Apple has exposed, and hence this is not cross platform and not easily portable.
That's why most libraries are easily ported (actually most are developed on Linux) to Linux but not their graphical shells.
On a side note: There are frameworks out there with which you can create cross platform GUI applications. Qt comes to mind. But the fact that those frameworks are cross platform also makes the applications created with them less user friendly on a specific platform than a ""native"" GUI application. Those frameworks tend to make everything look generic across platforms, which in the case of Apple is bad, because Apple created a very specific user experience which not easily ""fits"" into other platforms.
Edit (to incorporate the comments in the answer - thanks @Nick, @kbisset and @John):
A solution would be to port the whole OS X graphical shell (the closed source Cocoa/Core libraries - which is what makes OS X truly unique) to Linux. And technically, Apple could do that quite easy, but they have no reason to, as their entire business model is the uniqueness of their entire platform - hardware and software.
It's CONCEIVABLE that someone could attempt to clone the libraries, but that would take man-decades to do, and probably would never be right due to all the undocumented calls which would have to be replicated."

"The graphical layer is not the same, at all. OS X uses a proprietary graphical framework, linux uses X (X11/Xdotorg)

Almost all native OS X applications use frameworks such as Cocoa, CoreAnimation and so on, which are only available on OS X.

For example, say you have an application which stores a password for the user - on OS X, this would use its Keychain system and relevant APIs. If you were to port this to Linux, there is no direct equivalent, so you would have to re-impliment this entire feature. That is a tiny feature, and would require a large rewrite.

If you write your application using a cross-platform GUI library like GTK, Qt or wxWidgets, and porting will be much easier (or, ""possible"") - but the operating systems are still very different in terms of UI (for example, OS X uses a separated menu bar, whereas Linux tends to have its menu bar for each window)

One of the best cross-platform ports I've seen is
Transmission, which implements its main functionality as a library (which contains as little platform-specific code as possible), then creating native GUIs for each platforms, separately. This means each port shares a lot of code, but all have good, native interfaces (rather than a single interface which fits nicely on Linux, but is out of place on OS X, or vice versa)

There is a project called Cocotron which ""aims to implement a cross-platform Objective-C API similar to that described by Apple Inc.'s Cocoa documentation"" which would potentially make porting much easier, but there seems to be very little activity of late (the last blog-post was in December 2008)"