Maybe it's because young people use them a lot in messaging apps and older people are like "gah! young people and their picture messages! grmbl"?
And yet, I don't quite hate them - they can be useful. I just wish we had something better.
It makes communication MORE difficult and LESS clear. Text communication is a direct conversion(translation) of speech. You don't say:
"left arrow-jump-levitate-smiley face"
You do however say:
"I'm jumping for joy"
But consider the other possible meanings:
"Gravity is lower over there. Wee. This is fun"
"Did you see jump back there? WooHoo!"
"In the past I would practice yoga and meditation. I liked it."
... and i could go on. Because say it with me:
A picture is worth a thousand words.
Which of those thousand words depends on the individual decoding/defining/interpreting the message.
If you find you are unable to convey a clear, concise, unambiguous message using text; you need to read more and perhaps go back to school.
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.