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:
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.
Whereas within some CMSs it may take you 2 hours to install and get it looking as you want it, I would suggest that Joomla is really the quickest to set up. I would say around 10 minutes and you can already have something that loks as you require. Now if rapid development is your thing that is amazing!
SO really if you want to have a go, have no fear, with the amount you can download, and findsupport in the forums, Joomla, from my perspective, is the number 1 simple CMS (not that I am suggesting the ost powerful and configurable, but for ease of use and speed you can’t question it).