On the site we try keeping the information "under one roof", that means that if a question is a duplicate of an existing one we will close it as such.
When a question is closed as a duplicate of another there is an automatic mechanism that will redirect users to the main post automatically if they land on the duplicate via a direct link, ie: search engines and similar, but that only is true if there are no answers present on the post.
Since the information present on your question is already present on the main post's answers there was no need to move your answer to the main one.
Resuming: the answer you created is fine, but the question is a duplicate of an existing one, we closed it, we want automatic redirection to work so we deleted your answer instead of moving it since the information present in it is the same as the information present on the main post's answers.
Nothing personal or that you did wrong, its just clean up.