Drupal has gotten more difficult for 'ordinary' people to deploy. That's a big reason there are many times more sites still running Drupal 7 than there are running the current release: https://www.drupal.org/project/usage/drupal

The complexity compared with something like WordPress is a big part of the difference in install base. Drupal powers under a million websites (I run 3 of those). WordPress powers something north of 450 million websites.