Why is it Soifon didn't know about Shunkō?
The answer was stated in the same episode that it was revealed.  Yoruichi explained to Soifon that she did not want to teach Shunko to her because it was too unstable (if I recall correctly), with Yoruichi stating "Even I can't control it."  

Edit: I did not read all the comments under your question and did not see that you already knew that much, here is the rest of my answer;  This also explains why Soifon would never have witnessed Yoruichi using the technique, because it was unsuitable for battle.