Content management systems have to be very intuitive in content creation. Editors can focus on writing instead of broken buttons and bad formatting. Drupal is on top of CMS pyramid. This is why huge (not only) Czech media uses it on their websites (Czech Radio, Czech Television or for example University of Oxford).
Drupal has unique developers base, which counts to millions of registered users. If you have a problem and community is this big then there is a big chance that your problem is already solved by someone and much needed solution is available.
Speed and cost of Drupal development
Speed and cost is related to previous point. Thanks to modular architecture and rich offers of already built solutions (modules), there is no need to start from scratch. You gain already built system instantly and bend it to your imagination and needs.
In todays information society, security is a very hot topic. Drupal doesn't neglect security at all, it is actually one of priorities. Drupal association has its own dedicated Security team, which thoroughly tests security at every new release. Reaction time during security threats is minimal and a lot better then with proprietary systems. Also every published module goes trough security check.
Top quality design
Your client won't be intrigued by administrative advantages. He wants nice and high quality user experience. Drupal makes trends in this area. Using modern theme system Twig, possibilities are endless. You can simply create high quality design of whole website, which will support you in gaining high conversion rate and customer satisfaction will also skyrocket.