Q: How to delete posts in Apple Support Communities?

Not after 15 minutes has passed unless there is something that the hosts would deem a necessity, after being explained to.  I did answer your last post but you never acknowledged it.

Besides, the posts may also help others in the future if they were having the same problem.

-Ethan

Like Ethan said, it's most likely kept for people with similar questions (hence why the 'instant search'-like feature appears when typing a question. Apple may not want to compromise this feature by having people delete their questions, whatever the reason may be :/