Comment by apnorton

There are three buttons that act as sorting directions at the top of the answers section: "Votes," "Oldest," and "Active." The "Active" option sorts by most recently modified, which is _usually_ what you'd want instead of strictly newest. (i.e. an edit would update the timestamp, making that answer have a more recent activity date)

So, I guess the answer to your question of "why can't I" is "good news! you can" :)