What happens when a crate version has to be removed due to a critical CVE or court order (IP Law violation, perhaps)? There may come a day where crates.io becomes torn between not breaking Linux source and not hosting actively bad source code.
Note that some of those concerns do apply to vendoring source as well, but the additional download step also removes options that the kernel maintainers have as long as they ship all the source for the kernel in one tarball. Like more control over the timing of inevitable decisions.