Analogue: Why have more than one text editor when vi already exists? There’s no reason for any of the others to keep on going.

Yes, there are other ways of getting Windows software to run under other operating systems; WINE being the main one, but also virtualisation, building from source, or - loosely - substituting for similar. There’s only one that aims to be an OS in its own right.

You might call ReactOS the Emacs of Windows emulation. Everything else is just a text editor.