What other protocols and systems were you thinking of that can provide (depending on broker) hard-realtime delivery that can be used in safety-critical systems?
There's a very small number of major contractors and if you pick one of their APIs as the new standard, then you disadvantage all the others.
If you pick a greenfield project, then it will take a long time, and once you are done everybody will claim that they can't use it (see e.g. Ada; quick version they started working on a mandate for a programming language in 1975, mandated Ada in 1991 and by 1996 removed the Ada mandate).
Obscurity is part of the allure, not a repulsive factor.
You don't want a hostile nation suddenly and easily understanding your systems.
This is a principle in cryptography, but carries nicely to other things.
World of Tanks has repeatedly had military personnel updating the technical specifications with details they have found in the secret manuals.
Those documents are often secret or top secret, or even more compartmentalized.
There are training docs in wide use today that are not publicly findable on the internet, nor are they seemingly in the datasets of LLMs.
I'm sure there are reasons for that, of course.
What you're talking about would have an even higher level of obscurity and secrecy, and probably Compartmented Information access.
Often connectors are even proprietary.
(This is partially to protect systems not from the enemy, but from the troops themselves...)