A bunch, including:
1. Most of those platforms don't have compilers for any languages other than C(++). If the platform has a lot of history behind it, maybe you could write it in Ada, but that's pretty much it.
2. Development tools (debuggers, static analyzers, standards compliance verification tools and so on) for C and C++ are very hard to match, both in strength and in sheer availability. In the meantime, Rust still relatively recently got decent GDB support.
3. A lot of Rust's features simply aren't needed when writing this kind of software (e.g. the breadth of features related to memory management is largely unneeded because everything is statically allocated).
4. For better or for worse, C is well-understood (C++ is... well, not that I haven't seen good safety-critical code written in C++, but in my experience, C++ code is a lot easier to get wrong, both by humans and compilers). Rust isn't, not yet in any case. There's no Rust equivalent for e.g. MISRA, and not because Rust doesn't need one.
5. To, uh, put it bluntly -- C and C++ are very well known in the far corners of the world where a lot of this software is outsourced. Rust -- not so much, because outsourcing companies don't really encourage their employees to learn this kind of stuff.
6. There's a lot of commercial risk involved. I'm not sure about autonomous vehicles, this is probably a more volatile field, but many safety-critical systems have to be maintained for a very long time (10 years is fairly common, and 15-20 isn't unheard of). Rust may well be dead and burried ten years from now, whereas language enthusiasts have been singing requiems to C (on roughly the same tune as Rust, no less) for almost thirty years now.
Rust is a great development in this field and I can't wait for the day when we'll finally put C (and especially frickin C++, Jesus, who writes that!) to sleep, but it's at least five years away from the point where I'd even half-heartedly consider it for a project with critical safety requirements.
> If their code was written in Rust, that sort of bug could not have occurred.
I don't know the specifics of the bugs you mentioned, so I can't really comment on this, but in my experience, most of the similar claims that float around the Interwebs are somewhat exaggerated when put in their proper context. E.g. Heartbleed, which wasn't because C something something PDP-11, but because someone decided to be smart about it and implement their own (buggy) memory management system so as to make the damn thing run decently on twenty year-old operating systems.
I've seen people write that kind of code, for similar reasons, in Java and Go -- and, at least once, with Heartbleed-like results. The ways in which a language can be misused rarely reveal themselves before that language breaks out of its devoted community.
To be clear on it though -- I think Rust is a step in the right direction, and one that we should have taken a long, long time ago. If it can make it through its infancy, and if it can get enough commercial support, it will be a great alternative to C and C++.