Because of Plot. Kishi had to nerf Sasuke So naruto could shine.
-> Sasuke teleports 5 inches infront of kaguya's face while she's immobilized in a gravity zone. Kaguya has enough time to swap dimensions and fly off. Sasuke fails.
-> Naruto Jumps 50ft into the air and takes a swing at kaguya. Kaguya gets her hand ripped off even though she has a byakugan which sees 360 degrees and has precog to slow down movement. Naruto succeeds.
-> Sasuke swings his PS a few feet away from kaguya. Kaguya's suddenly a Russian gymnast doing all these jumps and rolls. Sasuke fails.
-> Naruto charges 9 rasengan and tosses them from 50ft away. Kaguya suddenly forgets movement or blocking exists. Kaguya stands there and watches the incoming fireworks. Naruto succeeds.
-> Sasuke dives down in a surprise attack to 1 shot Kaguya. Kaguya blasts his PS into Pieces. Sasuke fails.
-> Naruto watches kaguya in her face, screams for the universe to hear and throws a head on punch. Kaguya ends up in the side of a mountain. Naruto succeeds.
-> Sasuke consistently attempts to defeat kaguya with chakra based attacks (amaterasu, PS, etc). Sasuke fails.
-> Naruto tosses a chakra based volley of Rasenshuriken. Kaguya suddenly forgets how to absorb. Naruto succeeds.
-> Sasuke assaults madara multiple times with his S/t. In the spam of a few minutes he uses his S/T over 3 times. Sasuke's teleporting all over, teleporting his opponents all over and swapping with swords. Sasuke's S/T is OP as shit and his foot and handspeed are deadly quick to the point of garnering madara's praise.
-> Suddenly, Limits appear. Sasuke's jutsu now has a 1min+ cooldown after each use. Sasuke's suddenly as slow as sakura. Sasuke's sharingan precog apparently no longer works because he can't dodge for shit. Sasuke's got 99 problems.
Not a sasuke fan, but I feel sorry for him. With kakashi's new PS, sasuke's been made irrelevant as far as I can tell. He serves no purpose at this point in time.