Because you can is a pretty traditional reason.

Zig is supposed to be an improvement upon C, so doing C things with it seems reasonable.

Kind of neat that there's no need for a separate assembly file although there is inline assembly. Might get better (or worse) syntax support for separate assembly files? But it doesn't make a big difference until there's more features that need it (interrupts, threads/processes and maintaining their stacks, syscalls, starting other processors, etc)