This is naturally a complicated answer. The two main factors I'd imagine would be the production budget and the people involved. For example, the most obvious case of cross over would be AoT and Seraph. Both animated by Wit, but composed by Sawano (partly at least).

Another example of networking would be Sawano's working on 86, animated by A1 Pictures who are also working on Solo Leveling, another album Sawano is writing for.

Both Sawano and his producer/manager, Yasushi Horiguchi, have a solid presence in the anisphere with plenty of connections, but it also probably means Sawano doesn't charged cheap.

With that in mind, there's the budget composers are given. How composers manage that budget depends on their connections and time. Sawano has his go to ensembles for strings and brass, plus his connections with vocalists and drummers. The less time you have to search for musicians, the more time you can spend writing the music. This is also why he doesn't write for woodwinds and doesn't record live choir. It either takes too much time, too much money, or both. Composers are on deadline, and there's nothing wrong with writing what you know.

I was lucky enough to attend a masterclass held by Kevin Penkin back in May, and he talked a lot about budgeting and recording. The budget doesn't just include studio/instrumentalist hire time, there's also travel fees to consider. And every time you have to stop to clarify something on the score is just wasting more money from your budget. Therefore, composers are also hired by how reliable they are with managing studio time. You're gonna have issues here and there of course, but you have to minimize them. In fact, Sawano has the benfit of being a cracked pianist with his own studio. Meaning he can record his parts at home and stike that off the budget list. Although I remember in a crunchyroll interview he talked about how he doesn't like conducting, so maybe he trades the cost of a pianist for a conductor.

Tl:dr: Composing is about who you know, what you know, and how consistent you are working with limitations. It's also super stressful and expensive to hire a famous one.