I think it's because of the hardcore minimalism of the creators or Tachyons and also Theme UI, it lacks mass appeal because of it. If you dig into Steve Schogers style (the guy behind Refactoring UI and Tailwind aesthetics) you'll see that he's more in the Streamline Moderne camp than Bauhaus minimalism, his stuff looks more "designed" in the sense of how non designers think about good design. Tachyons is kinda invisible..
