Why?
As a summary, though:
PyTorch has become dominant in research because of its API (both its stability + having eager mode).
TF has become dominant in industry because A. it came out several years before PyTorch and industry is slow to move, B. It supported a lot of production use cases (mobile, serving, removing Python overhead) that PyTorch didn't for a long time.
BTW I've also read (here on HN) PyTorch learns much faster than TensorFlow does.
The Keras interface for tensorflow makes it easy & fast to make "good enough" models. That is often a driving factor
industry: tensorRT.