Is it though? I'd probably install GuixSD on a virtual machine if I knew most of the non-free software are used are packaged or at least accepted [1]. This means more users, which means more contributions and it's realistic for the project to go somewhere.
I can give you a baseless opinion I don't have any fact to show for but I don't think it's so hard to see: many GNU projects are very cool but for some reason they fail to reach critical mass and they just seem to grow very slowly (or die a slow death, depending on the point of view.)
1:
> [...] the GNU distribution follow the free software distribution guidelines. Among other things, these guidelines reject non-free firmware, recommendations of non-free software, and discuss ways to deal with trademarks and patents.
http://www.gnu.org/software/guix/manual/html_node/Software-F...