Prevents? There's nothing like that. The only something about git that hampers comprehensive porcelain alternatives is the lack of orthogonality in git's plumbing.
People spent years bitching about CVS before a few Apache-related people got together and wrote Subversion. The long time it took didn't point to anything fundamental about CVS preventing anything.
Perhaps this is pointing to the fact that there is something fundamental about git that prevents such a thing from happening?