the reason why is because itachi is probaply one of the best genjutsu users if not the best of the series,but sasuke already broke his strongest genjutsu technique with just his 3tomoe sharingan. this leaves itachi with only amaterasu,susano and ninjutsu,sasuke already has better control over amaterasu and a higher stamina. which shall eventually outlast itachi and make him win. so ms sasuke>itachi ems sasuke>>>itachi itachi>hebi sasuke though