Is CiviCRM already installed?
Even if the DB is clean, does the wp-content/uploads/civicrm folder exist with the civicrm.settings.php file in it. The installer does not expect that to be there.
You shouldn't hit the install page on an upgrade. I'd expect you to go to http://example.org/wp-admin/admin.php?page=CiviCRM&q=civicrm/upgrade&reset=1