Why drush is not maintained in drupal.org?
The motivation for moving to GitHub is discussed on our wiki page: 


  Drupal.org is poor fit for Drush as we get features we don't want on our project page
  
  
  Usage statistics (Drush isn't a module and thus can't report usage)
  Release names must map to a single Drupal version
  The issue to fix this stuff is nearly five years old.