It's not just that, Notepad++ is built around Win32 APIs and is designed for Windows. He's got some non-portable optimizations baked in. At its core, Notepad++ is just another Scintilla wrapper (like SciTE or Textadept) but it's targeted at and optimized for Windows. There will not be a Mac or Linux port.

If you want an editor with the same core as Notepad++, but fewer batteries included and more extensibility, Textadept is worth a look.