The Drupal 7 install.core.inc writes it's own configuration file, and…

… doesn't expect to be in a configuration bootstrapped state (as Drupal 6 did). Trying to bootstrap configuration with no configuration file causes issues with commands that come later in the request (e.g. quick-drupal), since DRUSH_DB_CREDENTIALS is left empty even though there should be usable credentials.

commands/core/site_install.drush.inc

@@ -91,6 +91,8 @@ function drush_core_pre_site_install($profile = NULL) {
         $install_profile_config = "\$conf['install_profile'] = '$profile';";
         file_put_contents($settingsfile, "\n" . $install_profile_config . "\n", FILE_APPEND);
       }
+      // The Drupal 6 installer needs to bootstrap up to the specified site.
+      drush_bootstrap(DRUSH_BOOTSTRAP_DRUPAL_CONFIGURATION);
     }
   }
 
@@ -102,9 +104,6 @@ function drush_core_pre_site_install($profile = NULL) {
     }
   }
 
-  // Now we can bootstrap up to the specified site.
-  drush_bootstrap(DRUSH_BOOTSTRAP_DRUPAL_CONFIGURATION);
-
   // Empty or create the DB as needed.
   $db_name = $db_spec['database'];
   $scheme = _drush_sql_get_scheme($db_spec);