Why would you think so? Nishino clearly had the best developement and scenes with Jumpei over the whole series, she liked him first she showed clear deep affection in a meaningful way.
Satsuki just eventually turned out to be extremely one-dimensional and underdeveloped character, whose scenes with Jumpei consisted of her just trying to seduce him by being slutty and then asking whether he liked her or not. It was completely flat relationship with few very brief expections including most noteably time where she tried to just be his best buddy, but that's not even compareable to the time that Aya/Nishino were given. She was also very manipulatitive and callous at times, showing herself as person with poor personality.
The anime isn't even over yet you still say Yui won?
We all know very well who the best girl is
How could anyone forget it?
Atleast someone in this thread has good taste.