Drupal is a huge software project by any measure, with thousands of developers writing code for it and deploying websites and applications on it.
As Drupal comes ever more popular, it will inevitable become more visible and targeted by hackers.
This may be a trivial case where user 1 (uid 1) can actually be deleted.
Joomla! is a well-known content management system, mature enough to be used by thousands of amateur and professional Web portals.