Rare Candy is a dead card in a deck like Greninja that absolutely wants to utilize Water Duplicates as frequently as possible in order to fill the bench. It's also why you don't run Brigette, because this deck isn't about relying on basics.
Rare Candy is only useful in a situation where you're frequently relying on going from basic to stage 2, whereas Evosoda is something that can get you to the next evolution regardless of what stage you're at.
Also, you have to have both a Greninja and a Rare Candy in hand in order for it to be played. With Evosoda, you only need Evosoda. Drawing for only 1 card to progress your board is far more consistent, and consistency is the main thing Greninja needs to focus on.
In Greninja, if I'm going to have room for 2 Evosoda/2 Rare Candy, I would always rather just run 4 Evosoda.
Rare Candy is only useful in a situation where you're frequently relying on going from basic to stage 2, whereas Evosoda is something that can get you to the next evolution regardless of what stage you're at.
Also, you have to have both a Greninja and a Rare Candy in hand in order for it to be played. With Evosoda, you only need Evosoda. Drawing for only 1 card to progress your board is far more consistent, and consistency is the main thing Greninja needs to focus on.
In Greninja, if I'm going to have room for 2 Evosoda/2 Rare Candy, I would always rather just run 4 Evosoda.