Don't know the answer. But I have a friend of mine that is a 5th Dan in Tang Soo Do and they still refer to knife hand as Sudo. Perhaps Sudo sounded too much like the Karate term shoto (sorry on the spelling if it is wrong). In which case could it have been a way to further differentiate ITF TKD. Or could it be a dialect thing. or could it be the actual striking tool or way it is executed?