To get a full understanding of a post and the discussion occurring after it, a user must first read the very top of the page for the original post, scroll all the way down to the bottom, and begin reading the page backward from bottom-to-top. Then, for any replies to top-level comments, the user must read from top-to-bottom, and then scroll backward until they find where they previously left off. This design makes for a very awkward reading flow. Instead, by sorting top-level comments in ascending chronological order, it'd be familiar and consistent and the user would simply read the page top-to-bottom.