Wordpress was created strictly as a blog platform, on the other hand, Joomla from its beginning was a full feature CMS. These are the main reasons for the differences between those two content management systems. WordPress comes equipped with several features built into Core, one of them is comment system. Although Joomla! 3.3 (3.4) don't have in-build comments, and probably even in future version 3.5 will don't get it, you have possibility to install & use them in a simple and fast way. And probably you have even more possibilities how they should work for you.