'screen' uses C-a as a prefix shortcut so developers of 'tmux' decided to use the next available key to avoid clash.
C-a does annoyingly conflict with readline's "move to start of line", though.
2. In outer tmux, ssh to some other machine you need to administrate.
3. screen because you need multiple terminal sessions on that machine, or need detachability/reconnectability for some long-running process.
Obviously they could have switched before a general release, but I guess they had gotten used to it.