Any writer has the freedom to what they want right? So oda can decide when to introduce any potential strawhat. And it's not as if it's oda who declared that he ll introduce every next strawhat at the first parts of the arc. I mean he can do whatbhe wabts
The point is that with every straw hat, Oda introduces them at the very beginning of the arc, or multiple arcs in advance of joining and he develops them at a decent pace alongside the arc(s). Doing this helps to make developments not feel rushed and last minute. The next straw hat is most likely the final straw hat and so Oda needs to try extra hard to make them feel integrated into the group.
Jinbe was introduced in Impel Down yet he only joined in WCI. He hasn't even had his official toast into the crew yet as that has been postponed to the end of this arc. We've known the straw hats for so long, so Oda needs to try and make the new straw hats feel like genuine parts of the crew and not just extra characters that have been added. That's why the fact that Jinbe has been with the crew on multiple journeys before he joined at least helps him to mesh into the crew naturally.
Yamato is a character introduced out of nowhere, very late into the arc with no connection to the characters or the readers and is immediately demanding to come on the ship. That's the exact opposite of what you would expect and opposed to what Oda is doing with other characters.
Look at Tama. A character introduced in the 3rd chapter of Wano, formed a relationship with Luffy, knew Ace, wanted to go out to sea with Ace and has been a recurring figure throughout the arc. Why should Luffy or us the readers care about Yamato, this random character he's just met?