Which platforms does Red Hat support Linux containers on that Rust doesn't support? Which platforms is anyone else frequently using Linux containers on that Rust doesn't support? I'm pretty sure the venn diagram here would be a single circle. Rust supports the major architectures.
Portability seems like an unlikely reason here, but I would pick memory safety over portability every single day of the week. Rust is far from the only language that offers memory safety, but it is certainly one of the most compelling ones for projects that would otherwise be written in C.
"runc" is battle tested, written almost entirely in Go, which offers memory safety, and the performance delta under discussion is only about 2x for a part of the system that seemingly only has a measurable performance impact if you're starting containers as quickly as possible.