Don't know about that one specifically, but comments are by their nature meant to be temporary on Stack Exchange. Their purpose is to ask or provide clarification on posts, and once they are no longer needed they get deleted, or if they are offensive, or even if there are just too many comments, as they make it harder to see posts etc.
So the answer is: because that is supposed to happen with comments.
Just quickly checked that specific post - 15 comments have been deleted, all because they have no further value and were just clogging up space making it difficult to read. Some of the others are good to go, so I'll be removing them shortly as well.