Because wp.com isn't running wordpress but wordpress multiuser, a different software. If you had the ability to upload plugins, since it;s a shared code base, you would be putting all of the users at risk if you upload an insecure one and they would be doing the same to you. Reference: http://en.support.wordpress.com/plugins/ Code (markup): You do know they have a support forums, right? http://en.forums.wordpress.com Code (markup):
Rep given. Plus the intention of Wordpress.Com is not commercial and most plugins have commercial intent even if it's just to improve your ranking.
because wordpress.com is a free blogging services.. Not only you can't add plugins, many limitation too like you can't add an iframe, can not upload/use new themes... If You want add plugin, i suggest you to install wordpress on your private hosting.. or if you only wanted free blog service with wordpress you may get a free domain names and free hosting services.. you may search it to the internet... google..
Wordpress.org and wordpress.com are different. Wordpress.org only provides software and pluggins. The site communicate the development of wordpress. Wordpress.com provides a service for blogging using wordpress software.