The main issue is mission assurance. Using the stack or the heap means your variables aren't always at the same memory address. This can be bad if a particular memory cell has failed. If every variable has a fixed address, and one of those addresses goes bad, a patch can be loaded to move that address and the mission can continue.
Where do you place the variables then? as global variables? and how do you detect if a memory cell has gone bad?
That may be a perfectly good solution in many embedded environments, but in most other context's global variables are considered bad design or very limiting and impractical.
For instance, how would better tooling help with storing a TCP buffer in global memory?
The first will be a real pain, as you now have 3 global variables, and the second will look pretty much like multi-threaded Rust running on a normal OS, but with some extra logic to handle the buffer growing too big.
You can probably squeeze more performance out of the C code, specially if you know your system in-depth, but (from experience) it's very easy to lose track of the program's state and end up shooting your foot.
So it's mostly about the absence of abstraction, in the C example? C++ would offer the same convenience (with std::mutex and std::array globals), but in C it's more of a hassle. Gotcha.
One more question because I'm curious - where would you anticipate C would be able to squeeze out more performance in above example?