Why is it Soifon didn't know about Shunkō? After Rukia is rescued and everyone is off fighting battles, during their fight Soifon says she developed a new technique which she only just perfected and hasn't even gotten a name. 

however Yoruichi says it does have a name calling it Shunkō, she even explains that the reason why the Stealth Force's attire is designed the way it is is because of how Shunkō works and that fabric on the arms and back would be blown away.

Yoruichi is Soifon's mentor and took her place as head of the Stealth Force when Yoruichi disappeared. if Shunkō was developed between Yoruichi and Urahara after they left then the design of the Stealth Force's attires wouldn't be so linked to it.

So how come 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.