It's old emphasis from at least the IRC heyday era (where I picked it up), likley earlier as some of the sibling comments allude to. Interesting, HN takes asterisk characters around text and parses it to italics like this, but I tended to know that as meaning bold font emphasis, with /this/ as italics.
It’s a computer-era simulation of the 100 (?) year-old convention of using underline to represent emphasis in typewritten documents.
Or maybe the same convention might have even been used before typewriters, to mark up manuscripts? (A manuscript needs clear instructions to the typesetter, because someone’s handwriting might not make the distinction between Roman and Italic scripts immediately obvious.)
Regardless, it's certainly an old habit