Drupal is pretty un-friendly compared to other website builders - like WordPress.
I think the popularity is due primarily to developers who can charge more per hour (not to mention spend more hours) building your website with Drupal than with Joomla or WordPress. Since they can charge more, they like to recommend it.
I've heard that it's more secure, more robust, and more extendable than other CMS programs. But I still haven't seen anything it can do that I can't do in WordPress except make clients more dependent on my services.