Because...
Indra's chakra + EMS = Perfect Susanoo (shown by Madara and Sasuke)
Six Paths chakra + MS = Perfect Susanoo (shown by Indra, Kakashi and lately Sasuke)
Sasuke needed to travel to Kaguya's dimension which evidently requires a huge amount of chakra. So he couldn't use his own chakra, therefore he needed Six Paths chakra. And since he needed to use Six Paths chakra, MS was just the requirement to use Perfect Susanoo, since PS was also required to transport everyone else along with him. EMS in conjunction with Six Paths chakra would have manifested something greater which wasn't the requirement of the time.