Your first reason is a bit condescending. KDE Plasma, Unity, Elementary, etc. all have clear design guidelines, and are all made by professional designers. They are not any less professional because you disagree with their choices / reasoning, and it's not clear that what makes Material Design work on mobile will translate well to the desktop at all.
I agree, and the reason is mainly lack of good design tools on Linux. It wouldn't take much though, perhaps there will be a Qt based design tool in the near future which will have a Linux port, and we'll see more designers (especially web designers) moving over.
If there was Photoshop or Sketch for Linux, and a lot of people could migrate. And no, GIMP or Inkscape don't cut it
