From the user registration agreement and guidelines:
we reserve the right to delete any message or user for any or no reason whatsoever, without notice or warning, at any time.
Why that is may be for any number of reasons: because a post is against one or more guidelines, because a post or thread is repritive and we already have 50,000 just like it posted, because a given post is a "me too" post in which a user posts only to say what has already been said, or something merely nonproductive entirely because we have to pay for our bandwidth, because of any number of reasons.
It is, however, highly unlikely that a thread -- rather than an individual post -- will be deleted when a user needs a question answered, leaving them in the lurch entirely. Even with duplicate posts, most users are redirected to en existing post or thread they can glean their information from.
But overall, we are not obliged or compelled to have to explain why any given post or thread is closed or deleted. That is the preogative of any of the staff or volunteers at any time, per their judgement, as clearly stated in the user agreement.