What I never see mentioned these days is the what I recall as the main point of contention between WordPress and Drupal: how the administration interface was handled:
* WordPress: separate UI for administration.
* Drupal: nearly identical UI for readers and administrators.
This turned out to be a lot more important than people realized at the time.

Having used Drupal & WordPress back in 2005/2006, at the time, it was not at all clear which platform was superior. Frankly, at the time, Drupal really seemed like the way to go.

There is one other technical detail which may have put WordPress ahead in terms of installed users: schema. The WordPress schema allows very easy updating. I can't speak about the Drupal schema directly, but I can say that Drupal upgrades (for me, back in the day) were tricky enough that I ended up not updating Drupal for one site.

Upgrade difficulty is why I no longer run Mediawiki. It might be trivial now, back then it was a major PITA for an inexperienced user.