Well, that sort of depends where you look. The rendering of a markdown file in GitHub or in GitHub pages uses traditional line breaks. Not having physical line breaks in the text of a paragraph is unfriendly to change tracking, so adding un-rendered physical line breaks is extremely useful for substantial pieces of writing that evolve. Many (though not most) forum postings have this property.
It’s only in more casual contexts like issues and PR comments that GitHub uses non-traditional (“GitHub flavored”) markdown where single line breaks are significant. I’m certain they did that as a concession to developers, who may quickly type a little code into an issue without indenting it or enclosing it in triple-backquotes.
So non-traditional markdown is really the more developer-centric choice. Nothing obliges you to break long lines when typing into a forum post, and IME most people want to see a blank line between paragraphs both where they’re typing Markdown and where it’s rendered, so there’s no advantage to the default setting for most people.