Not taking anything away from this specific project though as it's a hobby project. Just making a wider point about the calls for deprecating C and C++ in GNU/Linux.
I'm all in favour of a "rewrite it in $SAFE" but the risk adverse part of me feels like Rust is a combination of too young and lacking in formal guarantees to consider the massive undertaking we're asking here. Particularly when you consider this is going to be a multi-decade project and one that we can't trivially switch to another language if we regret that decision part way. To give context to that point: it's already been 30 years of C with Linux, longer if you count BSD or Unix. So the commitment we're asking from Rust is massive.
Maybe I'm being overly cautious. Unfortunately I've found the discussions on the topic are usually underwhelming in terms of people being prepared to honestly answer the hard questions about Rust's readiness for such a critical role. And that has left me feeling more nervous than reassured that the whole thing isn't just a fad.
But that's exactly my point. The OP asks, why not a language "better" than C, full stop. That's the wrong question. There are many languages better than C according to someone, but terrible to write a kernel in. Better than C is a terrible metric. The momentum behind rust isn't because it is simply better than c in some unspecified way.
To actually answer your question, i think its both political and technical. Rust is on an upswing. In any political movement,momentum is important. Ada is old news, it doesn't have that excitement around it that is critical for a political movement to galvanize its supporters. From a technical perspective, it seems like support primary comes from people who hate memory safety vulns. That's the impetus but lots of languages prevent that. The aspects of c that make it popular are (syntactic) simplicity and full control. Rust has really good PR to suggest that it does the memory thing very well, well still valuing the other factors that make c popular. Other languages tend to try and improve a lot of things or stray further from the c path, which make it seem like bigger switch with higher costs.
So in conclusion, i think rust does just enough to have a benefit people consider "worth it" (relative to switching costs) well not straying too far from c values which makes switching costs low. It also came at the right time, has momentum and good propaganda, which is critical for any political "movement"