What makes Drupal superior to other CMS's such as WordPress or Joomla?

A couple of things come to mind.

First, Drupal is extremely flexible. This is due to Drupal being designed more as a framework than a specialized content management system. Out of the box, Drupal seems like it doesn't really do much at all. However, this is by design so that your website can be configured with only the features that you need without the extra bloat. Check out some diverse high-profile websites that run on Drupal.

Second, Drupal is extremely scalable. This is both in the sense of being able to handle unforeseen loads of traffic and being able to be expanded later. Because of Drupal's modularity, you can easily add features to your website down the road with minimal re-development effort. This can be a lifesaver if you're on a tight budget as it makes it much easier to develop your website in phases as you can afford them.