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.
I use Notepad++ on WINE and it works very well, doubtless it could be done on macOS too.
why though it is open source the only problem the original dev has is that they are using his name and trademark they could name it something else and it will be okay.
If they'd named it something else, it would indeed be OK. We wouldn't be here having this conversation if that were a thing that had happened.
When there are no trademark issues then there are no trademark issues to discuss.
But they haven't named it something else, so here we are talking about the trademark issues that this raises.