The reason why?
It depends on your needs for a web-app. Hopefully you have seen how Moodle offers learning management features "out of the box." Drupal on the other hand requirs customization but is more versitile. Think of how a person creates a sculputre. Moodle already has definition and simply needs the paint whereas Drupal can take almsot any form you want.
But what if you want both?
Moodle with Drupal can play friendly with each other as well.
Thanks for reading my site on my top two open source web-apps.