Because Indra's Perfect Susanoo was equal to Ashura's avatar because they both died after they fought with them.
Sasuke didn't have perfect susanoo based on the fourth databook so he needed bijuus chackra to match with naruto's avatar which is the same as Ashura.
Madara's perfect susanoo was stated to as powerful or stronger than the bijuus.
Which means it needs this bijuu avatar to match it.
And if you do math you get:
Sasuke juubi susanoo=Madara's perfect susanoo=Indras's perfect susanoo.
Sasuke's susanoo may have better feats but it's defense sucks because bijuu chakra that Sasuke took from the bijuus does not have a physical form so it needs susanoo to stay which is only ONE layer.
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Madara's perfect susanoo was stated to as powerful or stronger than the bijuus.
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Sasuke juubi susanoo=Madara's perfect susanoo=Indras's perfect susanoo.
Sasuke's susanoo may have better feats but it's defense sucks because bijuu chakra that Sasuke took from the bijuus does not have a physical form so it needs susanoo to stay which is only ONE layer.
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