Linux

It's because

1. They're old and they don't want to have to spend time learning something new.
2. They spent a lot of time learning C and getting moderately good at it. They don't want that knowledge to become obsolete.
3. They currently don't know Rust, and don't want to feel like the thing they do know is no longer the best option.
4. They aren't the ones with the idea to use Rust, and they don't want to lose face by accepting that someone other than them had a good idea. Especially not some young upstarts.
5. Supporting Rust is extra work for them and they don't care about memory safety or strong types etc.

In order to avoid losing face they'll come up with endless plausible technical reasons why you can't use Rust in order to hide the real reasons. They may not even realise they're doing it.

Some of the reasons might even be genuinely good reasons, but they'll come up with them as an "aha! So that's why it's impossible" rather than a "hmm that's an issue we'll have to solve".

It's not just Rust Vs C. This naysaying happens wherever there's a new thing that's better than the established old thing. It's a basic human tendancy.

Fortunately not everyone is like that. Linus seems in favour of Rust which is a very good sign.