[HeyLaughingBoy]

it is very opinionated/rigid about the manner in which communication should happen

This could be a strong factor in its popularity. If things must happen in a certain order, then the behavior of the system becomes easier to verify. Ease of verification should never be understated in safety-critical systems.

[billfruit]

Yet the industry uses largely the C language, which isn't a model for safety or ease of verification.

[magduf]

It is, compared to other languages, because it's simple and deterministic. The #1 most important thing with avionics systems and software is determinism. That's why they even disable CPU caches on avionics systems.

[HeyLaughingBoy]

Well, nobody's perfect ;-)