Because you can not delete your own thread/topic. The admin disabled it for members. Obvious reason to stop scammers or people who want to delete the evidences. Deleting a topic will also delete all the comments on a topic which is kind of hurting for the users who took their time to comment on a topic.
Okay, this is a good explaination why does one could not delete his started post/thread. It is rightfully implemented this rules since there is a little bit problem with anonymity where one could BOLDLY express and even could be harsh to one another. To get the full content and idea one must quote the whole thread started by OP so as to let others know and compare if the thread was being altered from its real content.
You can lock that topic and move it to Archival.
By the way how to move a topic to archival? I have never tried to move my posts in that particular so one maybe could help about this?I will get this back for some answers. I might be moving some of my posts in the near future.