To confirm what amWhy already explained the comments were deleted by moderators, to be precise by now three of us (including me) were involved (though when you asked only two, still including me), and some of their own comments were also deleted by the original author of the answer.
The comment thread was a train wreck. The part in which you were involved became a hostile back-and-forth, in part about another question entirely, there was also some on-topic substance in between but presented in a form not really efficient to others. Plus, self-deletions made the thread confusing.
At first selective deletions were tried, I then decided to pull the plug entirely and delete everything. The old comments while harmless were not all that useful either, especially after the original answerer had deleted theirs in part, and deleting everything is both easier and sometimes better as it sends a more clear message.
You repost was by now also deleted. It is fine that you explained your reasoning for the downvote at the start. But it is not that essential to have this information around, and the poster of the answer had expressed disapproval of the way you approached the situation.
To put it differently, what you said is not that informative either. "I downvoted this because I think that the first isomorphism is not trivial."
is not clear, and to the extent it is informative it is not all that useful.
Right, somebody found the answer too terse, which is a valid objection to be sure, and thus downvoted. This is fine, but it is not essential to have this recorded either.
To sum up, the comments got a deleted because they created much more heat than light. If you think it is necessary to have more details there, please write your own answer to the post.