There are also practical downsides: information on the internet is often scraped, archived and cached. For example at https://www.hnsearch.com/ and http://hackerslide.com/

If deletion is allowed, it would be easy to add "undelete" from a previously scraped copy, or even a page for all deleted comments with previous user details. Both of these already exist for Reddit. In a way, deleting a post would highlight and bring it to the attention of people looking for controversy or accidentally leaked information.