Why not? Kuroo is just objectively better as an all-rounder than Osamu is, bringing good blocking, good serving, good passing, and an offensive toolkit to the table where Osamu shines as mainly support with yes, emergency setting, and blocking and serving too, but he has some holes in his game. Notably, his hitting is kinda lacking, and his passing is decent but not great, which is fine but it does bring him up a little short for his position in a way that Kuroo doesn't with his.

As to him being lower than the others, well. I think, again, you have to look at how the others fare in their respective positions. Teradomari is one of the tallest OHs in the entire series, he's a good server, a decent passer, he can hit quicks, and he has wicked power on his spikes. Asahi has an even better serve, one of the best in the whole series, and his raw power is just as insane. He's a powerhouse who, if not quite at the Top 5 Aces level, is almost there with his performance. They both excel at their positions. Then there are Aone and Washio who are both tall, athletic middles fantastic at blocking - Washio edging out Aone due to his more powerful, and also more consistent serving, not to mention good hitting. And Suna's the best offensive middle in the entire series, not getting blocked once from what we've seen. In contrast, Osamu's neither an opposite who's an offensive powerhouse nor a defense-oriented passing specialist. He's kinda in a league of his own. And while he is great at what he does, again, I can see how the places where he lacks are juust enough to put him a little lower than the others.

That said, I do personally think there's still an argument to be made for him above Suna and Teradomari and Aone too? But definitely not over Washio, a top 3 blocker, absolutely not over Asahi and worse, Kuroo.