Applescript is used for automation of already existing applications which will still be written in C++ or Objective C. Applescript, a proprietary language with no other uses, is just being replaced by a much more common language in Javascript so people won't need to learn a new one to automate apps. People don't generally write applications using Applescript and they probably won't be using Javascript, so there's little overlap.
In GNOME Javascript is being used to write the applications in the first place. There is total overlap between Javascript and C/C++.
People see the GNOME choice as both redundant and reducing performance of desktop apps while people see the Apple choice as simply using a common language instead of a proprietary one for scripting.