Photoshop is currently written in OS X’s Carbon library, and is b…
Photoshop is currently written in OS X’s Carbon library, and is being ported to Cocoa for the 64-bit version. Cocoa and Carbon are OS X specific apis, not currently available in any other OS.
If Adobe were to port Photoshop to Linux, they’d either have to rewrite much of it, or tweak the Windows version to work acceptably with Winelib, and compile it as a 32-bit linux binary. The problem with that though is that would leave Linux users with no 64-bit version, so really a full port to Linux is the only way, and probably an expensive one at that.