It could have been written in any other language that compiled to native code, if we had more options available.
1. the type that do something
2. the type that claim they could do it better than the first group (this is by no means limited to programming, it is for example pretty common in politics)
So please show me an OS written in Go or Rust or Ruby or Python or Java that people can actually use for day-to-day tasks.
Yes, C++ is very entrenched. That is a problem. We should be fixing that.
Now the historical accident theory is pretty obvious: it could have been an Algol, Fortran derivative instead of C, if only UNIX used that as a basis. C itself could have been designed differently, and if so would probably have different flaws and qualities.
Asking for a Rust/Go/Whatever OS is dishonest, because you know full well it will fail for reasons that have nothing to do with the underlying language's intrinsic qualities: even if that OS is great, nobody expect it to be so great we all have to switch away from Windows and Linux. Well, except maybe if we prove the correctness of the kernel, hence ensuring the absence of many vulnerabilities. Research is ongoing, I'd like to see where that goes.
Lots of Pascal, but Pascal really was an engineering constraint at the time if you were doing actual system programming. And back then, that was a serious consideration.
In the 80's using C on MS-DOS, was only seen by those that had UNIX at work or universities and wanted to work home.
It wasn't even C, rather SmallC or any other K&R dialect.
In any case, everyone that cared about performance was using TASM and MASM, writing everything in Assembly.
I go along with historical accident to account for Unix's success, though Richard Gabriel thought it was a case of Worse Is Better trumping The Right Thing.