The obvious difference is that AI has abundant use-cases, while Crypto only has tenuous ones.
Maybe there is added negativity considering it is a technology where there is clearly a potential threat to jobs on a personal level (e.g. lift operators were very negative towards automatic lifts).
Hacker News comment sentiment is not a reliable measurem of what the average hacker news developer thinks.
For one, only people who are very invested about something will post about it.
For two, many comments are probably not from developers and instead from fake accounts.
It does not seem surprising to me that both of these factors would be in favor of a more positive sentiment for crypto. People that like it seem to really like it and talk about it a lot, and there is a large financial incentive for numerous actors to create fake accounts and comments.