It was likely edited to remove the joke because that is superfluous information that's not needed in the answer. While we don't hate fun, editing to add information like that is frowned upon. We'd like answers to be, well, just that. Answer the question as clearly and concisely as possible. The joke was not needed and literally adds nothing to the answer.
It was locked because we had hit an edit war, and we don't want to have those sorts of things happening here. I appreciate the attempt at humor, but also understand that there was no need to continually insist the content be in the answer.
One of the features of Stack Exchange as a network is that anyone can edit other people's responses. This means we can all work together to create awesome content and improve answers/questions, make them clearer, all of that good stuff. So someone saw your joke, realized it added nothing to the post, and edited it out. It's an edit I myself as a moderator would also do, if I were the one that saw it, as the content does not improve the answer in any real way.