I posted a comment on this answer which I thought was quite constructive and did generate a good number of upvotes. I noticed just now that its gone. I'm curious what the reasoning was.
Yes, all of the comments on that answer were deleted because the comment thread got out of control. If you think your comment still applies, just go post it again.
Comments on answers are not intended to be permanent. They're intended to help clarify or improve the post they are attached to. Ideally, if your comment was genuinely helpful, it would have been edited into the answer and no longer be needed.
Since deleting comments is a moderator-only action, and policing all comments on every question and answer would require an army of them, most of the time comments are left alone. But if they get overly out of control, other community members will typically flag the post for moderator attention, and most of the time the moderators will just clear the whole thread. Unfortunately, when a moderator does intervene, occasionally a relevant one gets caught in the process. This is just part of the disposable nature of comments.
This particular answer came up in chat when the comment thread got over 50 comments long, and it was handled. The comments that were currently on the answer are just the ones that have been posted after the previous set were deleted.