Because Kishi is trying to portray an ideal that most people on this site don't get or understand. Think about what Kishi has been doing in the Kaguya fight. Sasuke has been bad mouthing everyone and saying they would be more useful in infinite tsukiyomi. He is only fighting for himself and is basically being selfish.
Now think about the event surrounding the other characters. Naruto is fighting for everyone, fighting with the tailed beasts and defending his friends. As a result he is actually fairing pretty good against Kaguya. Sakura used the power of her Yin seal to allow obito to cross multiple dimensions and in the process saved Sasuke from the desert dimension. Obito managed to sync with kaguya's dimensions, saved Sasuke, saved Naruto and saved Kakashi. Now Kakashi is fighting to protect everyone and the memory of Obito, he just ripped out a PS and has dual Kamui.
Notice the trend here? Everyone that is succeeding in this fight is fighting for other people or better yet fighting to protect people. Sasuke won't succeed because he hasn't yet realised that the power of fighting to protect the people important you - a major theme in the manga. But people don't seem to get this because they are to busy with power ups and 'feats'.