Because Itachi wanted to stop Madara sama and Sasuke didnt want to see Itachi fail. Sasuke viewed Itachi as the embodiment of perfection and therefore how can he join somebody his brother tried to stop.
Itachi could have stoped Madera him self. I get the whole Itachi single handed won the war by stoping Kabuto something that if not done would have made the war rage on endless. with the only ninjas of caliber who mattered being Kakashi, AO, and shikamaru's dad. the 5 kage beating the (edo kages who was trying to be sealed) was nothing to madera who bodied all of them.
Kakashi with Might Guy, Lee and Sakura and the 1st devision beat the 7 Swordsmen.
Itachi single handed stoped Kabuto
AO was the sensor that was doing all of the work.
Shikamaru dad was the one to come up with plains
That being said Kakashi was the only one who could have beat Obito.
Madera is beat by Itachi or Might Guy.
War over.