That's because underlining is inferior typography. It's from back in the typewriter days when the only way to add emphasis was to back up and put underscores underneath the text. Either bold or italic is more preferable.
Historically underlined content == actionable hyperlink; for content that is intended for public consumption (ie. not a note that you're writing for your own use) there are usability issues with typographic styles that represent emphasis with an underline.