Because it's simpler to grasp than vi, at least for newbies. You hit backspace, it deletes a character. You hit enter, it makes a newline. And so on.
Once I got more comfortable with how things work I was confident enough to use it and I would not want to miss it any more.
Oh man, I remember googling that years ago now. That was fun. Now I use Vim and Visual Studio Code for all my editing, but because my Vim is so customized, if I remote into a server I'm more likely to reach for nano instead.
Its a terminal editor that uses hotkeys that you more commonly find in GUIs.