Siraj Raval covers PyTorch very well in a 5 min video (https://youtu.be/nbJ-2G2GXL0).

It comes down to design decisions, of which I'm not qualified to go into. This article made the front page last week, about the downsides to Tensorflow which people rarely talk about: http://nicodjimenez.github.io/2017/10/08/tensorflow.html

And this interview with a Tensorflow engineer (10 mins) explains a little bit about those design decisions (https://youtu.be/axRHotkkTVI).