It wasn't downvoted - rather, the account is banned (https://news.ycombinator.com/item?id=42653007) and comments by banned accounts are [dead] by default unless users vouch for them (as described by zamadatix) or mods unkill them (which we do when we see good comments by banned accounts).

Btw, I agree that that was a good comment that deserved vouching! But of course we have to ban accounts because of the worst things they post, not the best.