Jinbe got to stay behind so his powerlevels at the time of his joining doesn't "disturb" the dynamics.
That's why Oda had to wait for Enma and Raid Suit (Jinbe came only after both powerups established).
Of course, this reason can be nullified if Yamato joins, since she's stronger than Jinbe & current Sanji.
Maybe, but I have two issues with that. First, Oda was in total control of when his characters would receive which power up. He wasn't beholden to make Sanji wait until Wano to use his new suit, or make Zoro have to give up Shusui, a strong sword in its own right, for Enma. They were good story choices, but I don't see why he would have to trade off Jinbe for them. He introduced Zoro's fire cutting power just now, long after Jinbe, and that changes the status quo as much as Enma does.
Second, the first two acts of Wano were dedicated to staying low. Power ups are nice to get in the downtime, so would Jinbe really have shaken things up more than that? He likely couldn't go out into the Flower Capital, but neither could Brook. He's a lot more than a big strong guy, he's smart and focused, not to mention humble. Just like in the tower with Sanji, he's the last person who wants to disrupt the crew's dynamics, and he wouldn't just by being there.
The question is, what was Oda really playing at keeping him away for so long?
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inb4 someone mentions that Yamato showing up with Momo and Shinobu (who she was told to protect by Luffy) means she’ll stay on Wano, as if the mere sight of her with native citizens of the island means she won’t leave a place where she’s been trapped for 20 years 🙄
As far as we've seen, he was trapped on Onigashima, not Wano's mainland. It's possible he'll still want to leave, but it's also entirely possible he'll want to stay for Momo now that Yamato knows he's alive, which he didn't know all those times he said he wanted to leave. At the very least, Yamato won't have to stay in that same skull he's been trapped in.