Koichi is the narrator. It's different. And Josuke has much more of a journey. Jotaro is just totally closed off from everyone around him and doesn't really let the audience get inside his head very often. We slowly learn about why he's like that and how it doesn't mean he doesn't care, but we learn from the outside. This is not bad, and he's still more important than Pol, but we're nowhere near as close to him as we are to other jojos.
Part 4 is much more of a thing where everyone has their own Polnareff style threads they're following and we dip into and out of them. Everyone, including Jotaro. When everyone has a fairly equal spotlight, determining the protagonist by screentime becomes even less relevant.
To make another comparison: Gyro Zeppelli is not the protagonist of Steel Ball Run. While he's a lot more expressive than Jotaro, he bears a lot of similarities to him in terms of his dynamic with Johnny, who is the Polnareff of Part 7, and also explicitly framed as the part's protagonist by virtue of him being the one with joestar in the name.
It feels like an attempt to explore more intentionally a dynamic that happened sort of organically in part 3, and making it fit with the typical structure of the series.
EDIT:
Also, forgot to mention, every time i get into a groove with reading Jojolion something like, for example, a 9 year old boy feeling up and licking a grown woman's inner thigh starts happening for no reason. Also I don't think I posted about how yasuho's first time being attacked by a stand led to her almost getting raped twice, then drugged with sleeping pills and waterboarded, all in the span of about 20 minutes in-world. Her stand is super cool, the design is cool, she is cool, but everything the story is giving her to do these last few chapters is a huge bummer. Don't get me wrong, what she does with Paisley Park in this specific chapter is cool, but she was just in a drug-induced sleep for the entire last arc so i need more than just that.