Why does the download page recommend the previous version? Why does Drupal download page recommend Drupal 7.x? I can hardly see the footprint about Drupal 8. 

Does it mean Drupal 8 is still under development? Does it mean Drupal 7.x can be the last stable version?
Yes, Drupal is under development. See
 http://drupal.org/core/release-cycle

API freeze was in July, now API completion phase.