Surely the biggest thing has been accessibility from the start?

It’s all good and well to hear about Ada but how did one start using it for free at home to learn and then evangelize it in the work place?

How did one find learning materials about it in the same amount as an openly available language?

How did one contribute back to the language ?

Lots of cool stuff happens in proprietary spaces first. OSS variants tend to gain traction very quickly though

See https://ada-lang.io/ and https://learn.adacore.com/

I think it being open source from the get go has also made it a lot more viable.

That's made it accessible to amateurs, but cost is less relevant in safety industries than reliability.

It’s relevant in terms of there being a skilled developer pool to hire from. There are exceptions, but the vast majority of software technologies that have been success have done so by being accessible to hobbiests to install locally, even if that’s not their primary use case.

The good news about professional programmers is that they're capable of learning. Ada is not a hard language to learn, if you have some familiarity with procedural programming (which 99.99% of programmers do). Considering the amount of time spent on safety-critical systems, there's enough time to train up a new hire that only knows one language on Ada, assuming they're actually a competent programmer.