Quite simply because Joomla is just as easy to use as Wordpress, but it does not limit your scalability for future growth. The Joomla CMS and Platform offers rich features that are equal to and often superior to those in Drupal but because Joomla CMS has such a large Global community of developers and service providers you do not have concern yourself the complex nature and higher cost of managing a Drupal site. Joomla sits right in between these other top contenders as the best of both worlds and currently powers over 2.8% of all web sites on the internet.
We have found that the trade-off between a little more learning curve in Joomla vs Wordpress, which is easily overcome with a little training, offers our clients a much better long term solution. It ends up being less cost to maintain, you can do so much more with your online tools and the benefits of working with a service provider to compliment your skills and expertise through the process of building any web site can bolster your potential for success.