Obviously, the question as to why people are using it is one that can be prompted with a number of answers, yet I will put my few points down:
- The backend/administration is understandable
This can be a big problem, especially when using CMS for no IT professional, which I guess I would class myself as. Joomla has a relatively smooth and easy learning curve to being able to publish articles and create users/emails all fro its backend. Especially in contrast to Drupal which can require slightly more knowledge.