There were no good options previously. It was either C or C++. Most of the other languages were either fringe or had a GC, or had a pseudo runtime GC (Swift). The culture of Java and C# and Go didn't really support the type of low level optimizations needed, even though you could technically do system programming if you restrict yourself to a specific subset of language and cut yourself off from most of the standard library and ecosystem. Nim was unstable. OCaml had the same issues as Go and Java and C#. You simply did not have any options until Rust came along. Oberon was an academic trinket. The less said about the various lisps and forths the better.

OS and embedded programming require bare metal support and data structures that can run standalone in the absence of an OS and standard library, and the ecosystem must exist to support such a style of programming.

Currently Rust has over 10000 crates that would theoretically work just fine in an kernel environment.

https://crates.io/categories/no-std