Because if you started the thread and you delete the post it will delete all the replies to your post. We had instances where a user didn't like the direction a thread turned so he deleted the thread thereby deleting a bunch of peoples responses - who were then pretty irate that their posts were deleted. So we don't allow people to delete a thread that they started - if it's a reply you can delete it no problem. If it's one you've started and you feel it needs to be deleted then ask the moderator of that forum to do it.
I'm also moving this to the website forum.
-Steve St. Laurent
Lead Moderator