Back when most writers used typewriters, the convention was to use the underscore to mark text that was to be italicized. And because they were using typewrite

Back when most writers used typewriters, the convention was to use the underscore to mark text that was to be italicized. And because they were using typewriters, they would backspace to the first letter of a word or phrase, then type the underscore under each characters to be italicized. When computers came along, backspace worked differently (back up, replace character with space), so the convention was to start and end the word/phrase with an underscore, _like this_. Using asterisks for bold is again an adaptation for computers. With typewriters, you could just back up and retype the word/phrase to get a bolder look (maybe a few times to really make it stand out).