As most language decisions for new projects, it probably came down to "it's their favorite language".
> btw, Python programmers have the option of pymc3 https://docs.pymc.io/
And Edward (http://edwardlib.org/), "Pyro" (http://pyro.ai/) and "ProbTorch" (https://github.com/probtorch/probtorch).
The first built on TensorFlow, the latter two on PyTorch.
Stan (http://mc-stan.org/) also has python bindings: https://pystan.readthedocs.io/en/latest/