Earlier quoted context omitted.
While I understand why this change can be annoying to the tech savvy folks of hackernews, it can be quite helpful for those who aren't that tech savvy and want easy ways to bold, italicize texts (that make up majority of the slack user base). Heck I am sure most of the slack users wouldn't even know it was possible to bold, italicize on slack using markdown.
Why does anyone (tech savvy or otherwise) need to bold or italicize anything. In my mind bolding/italicizing are nice-to-haves alongside emoji. You wouldn't compromise the entire functional user experience of the central feature of your app for nicer emojis. (at least I hope not).
That's why tech people italicize stuff.