@Midday, We will never get that explanation because Sasuke already gained Kyuubi's Chakra.
The question is why should Hashirama's cells or Kyuubi's Chakra be the reason, the user can let his Susanoo grow legs, when it is simply Chakra that his shaped into a form? Kishimoto indicated that Madara could use that form of Susanoo at his prime, saying that everyone who sees it dies. Tsunade on the other hand asked herself whether her grandfather fought that Susanoo. When Madara's Susanoo armored Kyuubi, it also had legs present [
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The perfect Susanoo means Kantenzai Susanoo. That means, that Susanoo has a accomplished/complete body. When Madara activated his Susanoo, Mei said that this would appear to be the real Susanoo, whereupon Madara refuted and said that the Chakra first has to stabilize.
I also don't think that Kyuubi's Chakra has a significant influence on Sasuke now, as Kishimoto portrays them as equals. That would contradict the portrayal, because it would mean Sasuke can't achieve his ultimate form without the help of Naruto.
@Bogard, that's if they fight Obito, whose only weakness is Senjutsu. But I think they would be equal if they had to fight each other. A perfect Susanoo with Enton is quite a haxxed thing.