I will give you some of the reasons and compare those with Carrot:
Reason 1:
Each strawhats were changed by their adventure with Luffy or because of Luffy. This is a fundamental narrative rule. Right now, Yamato has yet to be changed by the current event or Luffy's actions.
Carrot innocence on the other hand, has been demolished by her journey. Meaning that there is the start of a change for her.
Reason 2:
Yamato has no dynamic with the crew. It's necessary to have a good dynamic with the crew as a strawhat.
Carrot on the other hand had clearly an entire dynamic with them.
Reason 3:
Everything in Yamato's current character arc is pointing to her being a protector of Wano. Yes she stated that she wanted to sail with Luffy, but her actions are making an opposite statement : Protecting Wano.
Carrot on the other hand may have almost no development, but her storyline is still arched toward the premisse of becoming a pirate
(cf chapter 1006)
Reason 4:
There is STILL no setup regarding the future post of Yamato on the ship. Oda could easily do that, even in a war, but right now there is nothing, no setup, no presentation of any capacity for any postwhatsoever. The thing is: ALL the strawhat have a postand those shadowed early in the story and the introduction of the character.
Right now, the parcimony principle must be applied to Yamato: She has no place on the Sunny.
On the other hand, Carrot was litterally introduced as lookout in the story. Her role was defined clearly later on and this even led to Carrot having a Shining Nakama action, another parameter that Yamato does not have.
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Take note that everything that I stated here could change for Yamato or for Carrot. But at the current state of the story, not only Yamato has small chances to join the crew, but Carrot is still higher is the probability board.