It's what screen uses by default, and a lot of people come to tmux from screen so they already have the muscle-memory. Not sure of the original logic though.

A lot of people aren't used to the Emacs bindings. Home and End are still widely used.

Really annoys me when I have to work on a server that has the Ctrl-A bindings in tmux though.

You set prefix to C-a and then send double-presses on.

So C-a-a still does beginning of line.

Control-s/e/d/f/whatever you want then. The point is to avoid having to reach all the way over to 'b'.

Probably because that's how screen did it before tmux