Why is the drupal.org site not using the more recent Drupal version?
Drupal.org is huge website the migration can take some time. The migration from Drupal 6 to Drupal 7 was finally done 2 years ago and it took forever. Moving forward to Drupal 8 is going to take a lot time. I do not see that happening like in 3 to 4 years.

Moving to Drupal 8 is not a higher priority right now for Drupal.org, compare to adding new features and improving the current website.

Drupal.com is a smaller website is been running Drupal 8 since Beta.