Earlier quoted context omitted.
Julia isn't GPL, it's MIT. Currently includes a few GPL libraries in the default distribution which makes the overall thing GPL, but that won't be the case for much longer. You can leave those libraries out for a no-GPL build.
Point is, Julia is by default GPLed and the GPL is not The Cancer That Is Killing Julia (or Octave or R). Without the GPL, Julia doesn't have FFTW, Suitesparse, or GMP.
A lot of people can get by just fine without FFTW or SuiteSparse (they aren't dependencies of everything written in the language, so don't belong in the standard library), and GMP is LGPL so not quite as problematic. But people do want to use a programming language to deploy applications, including proprietary commercial ones. Doing that in R or Octave isn't really possible because of the GPL, and that hurts the prospects of commercial backing (as does being pretty bad languages for developing software, but that's a separate matter).