It's because they're easy for me to remember and easy for me to type. No other reason. a=awk, c=git commit, g=go, r=replace, s=sed and u=undo (obviously z would work just as well for undo). The core idea was commands in the editor is just any executable in your system instead of a custom scripting language - kind of like CGI. You are free to implement commands in bash, tcl, javascript etc.
