I think it's industry focus in my opinion.
The companies selling C and Ada compilers (say, to Aviation or Medical), are not willing to risk the losses they would incur should they make such a stupid decision. Those same companies (Wind River, Adacore, Green Hills, etc) sit on the design boards and guide the development of the languages.
Not sure any company is paying the same (or really anything) for Python, which gives the community carte blanche over its development. It's a Catch-22. No one is going to spend big $$$ on a tool that can't nail-down it's standards, but then there's no reason to nail-down any standards if you're essential free to do what you want with it.
In other words, the people making changes to C/C++ and Ada have to consider what their customers want out of it. The people making changes to Python are only considering what they want out of it.