At the very least, I would suspect that particular bit of negative reinforcement failed at its job. A number of other features (karma primarily, but also the comments page and saved stories and saved comments) seem to encourage long term engagement far more than non-folding threads could discourage it. That downvoted comments alone appear on /newcomments practically makes it a flamebait generator. Tribalism, trolling, vindictive nastiness and even meme posting abound here.

On the other hand, I suspect (with nothing to back it up) that pg simply never considered thread folding, or else didn't consider it interesting. Meanwhile, any new feature will inevitably garner criticism from users if it doesn't work they way they expect, doesn't support their browser, etc. And, anything that causes Hacker News to look and behave superficially more like Reddit will cause more than a few users to rend their garments, put on sackcloth and ashes and wail in agony at the death of Hacker News.

That headache, combined with the 'unique' (and not really open) nature of the codebase, a staff with better things to do most of the time, and a strong cultural aversion to making even small changes in the UI, means otherwise apparently simple changes probably take far more time to roll out than many developers would consider reasonable.