zlacker

Here is what the Unix manual from Bell Labs (the origin of Unix) says about /usr/bin versus /bin:

    Commands generally reside in directory /bin
    (for binary programs). Some programs also 
     reside in /usr/bin, to save space in /bin.

There were hardware limitations in the early days of Unix. Short command names (like “ls” and “rm”) were used to save memory and paper (teletypes were used in the days before video terminals). Each byte mattered back then. There must have been a constraint on how much they could put in /bin (I am guessing).

Here is the manual:

http://plan9.bell-labs.com/7thEdMan/