Probably because once you start a thread others can post in it. If they post pertinent information and you decide to delete the whole thread that info is lost. So potentially you not only delete your content but someone elses.
Yeah, if you guys hadn't replied already, I might have deleted it. But now I am wondering if I should leave it here, so others can read your explanations.
You can always "report" your own posts / threads for us mods to jump into action.