Practically all of the most popular and widely used deep-learning frameworks are implemented in Python on the surface and C/C++ under the hood.
I think the main reason is that Python is widely used in scientific and research communities and it may be easier to experiment with new ideas and code prototypes quickly in a language with minimal syntax like Python.
Moreover there may be another reason. As I can see, most of the over-hyped online courses on AI are pushing Python because it is easy for newbie programmers. AI is the new marketing hot word to sell programming courses. ( Mentioning AI can sell programming courses for people who can not even write even a Hello World! :)