Wagtail is built on Django so it's easy for a developer to customise it for their own needs. It also allows you to easily create a website/webapp hybrid. There is not a lot of good CMSes built on popular web frameworks.

Wordpress has very little base functionality in it's API. Plugins consist mostly of vanilla PHP, so it's more difficult to extend. It's like what web development was 20 years ago. On the other hand it's great for non-developers because of the amount of the plugins and themes available.