Our organization uses Debian because we need an OS, and we like FOSS. That's why we chose GNU/Linux in 2002 (we had tested Linux several years earlier: Mandrake, Red Hat ... in dual boot). Since 2002 our office computers have only been running Debian, removing MS Windows from them. Some years later we started to maintain our own servers, and for that purpose we chose Debian / Ispconfig.
Why Debian and not another Linux distribution? Due to the same reason we chose Drupal over other choices to develop web sites in: its community ruling system. We like FOSS and community projects :-)