Why is Pressflow not listed under Drupal.org's List of distributions?
Press-flow generally takes the established features of the next version of Drupal and back-ports it to the current version.

For example Pressflow-6 Takes the features(enhancements and performance improvements) from Drupal-7 and back-ports it Drupal-6. These changes generally include changing the core to considerable amount. 

The only reason I think of why it is not included is that 


  Since it makes changes to the core it doesn't make sense to host it on
  the Drupal.org under project.