Or perhaps it's because sometime the meaning of emoji can be hard to understand, rather than "gah! young people"
Even if they manage to convey a meaning, like the other poster said, that meaning is emotional instead of factual.
Why would anyone ever wish to express emotions in a commit message, or even in a business communications tool like Slack, is beyond my comprehension.
For personal communication they can serve a purpose, but that's not the topic here.
Also another issue is that emojis are not as easily indexable/searchable.
So, no, I don't think the issue is that they don't work in this case.
HN definitely does have a knee-jerk hate for them, and if you spend enough time here you'll see that a lot of it really is just "gah, young people." It's the new hate on txt-speak.