I would imagine it wasn't a hard decision - they likely needed to build something in an environment where they are already paying 40-80k C++ developers, and I would guess something like 1-10k Rust developers, who are scattered around various teams and may not want to hop on a new team right now. Also it was released to the consumer market in 2018, so probably built in 2016-17. Rust and other memory-safe systems programming alternatives didn't have nearly the same uptake back then - so maybe 1-2k would be a safer bet. Not to mention, Rust didn't get tier-1 ARM support until 2021 - and even then, that's only when running on linux, which the Titan M chip is most likely not running.