By using shadow clones, he would be splitting his power up into weaker clones. That technique only really works with a nearly unlimited chakra supply, like Naruto's.
Except Nagato has one of the biggest chakra reserves in Naruto.
According to what? We have no data book stats on him, so that's mere speculation.
Not enough chakra
Except Nagato has one of the biggest chakra reserves in Naruto.
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Doesn't matter if he has a hue reserve of Chakra. The Shinra Tensei would weaken according to how many clones there are.
E.g. If he makes 5 clones and they all use Shinra Tensei the power would divide by 5.