But I suppose that doesn't really answer your question. Linux is/was a compendium of a bunch of little tools you string together. Many/Most/All of the items in Notepad++ can be found as smaller independent tools.
The real addicts will use console tools like VIM or EMACS to do their editing (not really GUI, but powerful).
As an alternate alternate, try using Visual Studio Code (VSC). Not quite as light weight as Notepad++, but very functional.
On Linux, I'll use KWrite to do my basic editing. I'll revert to VSC when heavy editing is required.
https://apps.gnome.org/en/TextEditor/
https://github.com/elementary/code
Since you're coming from Notepad++, maybe you are looking for something ugly? Try this:
IMO the answer is the lack of good UI toolkits for linux, with any sufficient definition of "good". GTK or Qt are probably the closest solutions to Win32 or WPF, but I think we can empirically just say that they are not "good" enough. If they were, more complex GUIs would exist.