Because all Naruto has here is FRS which honestly is an overrated jutsu.
People think that Sasori merely controls puppets and has no speed, strength or anything in his puppet body.
But really he was able to dodge a speeding huge block of IS coming at him completely and without any problems
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And his reactions are extremely above par, being able to force his core out of his chest before the bullet like seal engulfs his body?
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I mean the location is an open outdoor area so all he needs to do is react and jump the same way he did to avoid IS and the FRS is avoided. Combine that with the fact that if Naruto comes too close IS will pummel him before he even gets to throw it and Sasori has a relatively not too hard time of avoiding this jutsu. After he see's the jutsu once he will no doubt keep a block of IS near him to block the jutsu if the time arises. As I doubt it can tear through the IS.