There is not a lot of incentive to create a free Notepad++-like text editor for OSX. OSX already ships with two excellent free text editors for the commandline (Emacs & vim) and there's GUI versions of both.
Any Cocoa/OSX specific GUI editor will suffer from being less powerful, less well tested and less portable (whereas vim/emacs are available on any imaginable platform, allowing you to keep your editing muscle memory).
Vim and emacs are not for everyone. This may come across as sacrilege to some, but I personally don't like keyboard-driven interfaces. I want a mouse-based ui, with shortcuts for the stuff i do often enough. I want to move textblocks via drag and drop. I want to trigger a refactor command by navigating a menu with the mouse. And fundamentally, it's because my memory for remembering keyboard commands isn't good enough. A mouse-driven ui lets me focus on the code, instead of "what was that sequence of keys again?"
Sure, I'm just pointing out that their existence greatly diminishes the motivation to write a good (free) text editor for OSX. There are plenty of nice GUI editors for OSX, but most (all?) seem to be paid editors.
The end result is that you either fork over the money (not entirely unreasonable for a tool you use so often) or learn to work the excellent free editors that exist.
Any Cocoa/OSX specific GUI editor will suffer from being less powerful, less well tested and less portable (whereas vim/emacs are available on any imaginable platform, allowing you to keep your editing muscle memory).