Drupal7 was launched in January 2011. About 7-8 months later, we are now able to practically use it for web projects. Important 3rd party modules such as 'Views' were now available & stable in Drupal7. So any web project that go for a complete revamp/re-design of existing drupal sites also look for upgrading to Drupal7. New drupal projects naturally look to use the latest major version available.