Drupal 7 is a powerful platform which attracted a significant amount of website creation investment. It was a natural evolution from the previous Drupals.
Drupal 8 departed significantly from the previous Drupal philosophy and architecture. Converting a website to Drupal 8 requires a significant investment of resources. Drupal 8 is also naturally heavier and slower than Drupal 7 because of its incorporation of a huge base of PHP object infrastructure. Making and maintaining a Drupal 8 website is involved and requires an army of site design, backend, dev-ops, and more personnel. It is not for the faint of heart.
Converting a website from Drupal 7 to Drupal 8 is so difficult that (as of November 2019) not even the Drupal Organization has converted its own Drupal 7 website to Drupal 8 after more than four years of opportunity.
Drupal 8 also introduced a new dependency on an enormous project: Symfony. With this dependency, as Symfony develops, Drupal must also necessarily migrate forward. This means that as Symfony moves into version 4 and Drupal moves to version 9, all of the investment in Drupal 8 sites must actively convert to the new Drupal 9 and new Symfony. If this active maintenance does not occur, Drupal 8+ sites WILL become security risks.
For these reasons and others, the maintainers of the drupal.org, drush.org, and other Drupal-oriented websites have opted not to use Drupal 8.
Without such huge external dependencies, Drupal 7 and YAD7 have much lower administrative overhead, so an investment in YAD7 is truly an investment in a low-maintenance website that will last.
In short, YAD7 is for those who think it should not take a million lines of code to spit out a web-page and that more complexity leads to higher maintenance costs and lower security.
