Why becuase of many good features it have. Using kunena you dont have to hack or modify any core files. Just install and its ready to use. No integration bridges as it is made for joomla only. Very good support at the developer website in the forums, sometimes i get response in minutes. Its GPL and can be used accordingly. Its non commercial and a helping hand towards poor joomla users. Its under development and developing extensions are good to use. I dont see any shortcomings except one i.e. no html in posts.
Overall good extension.