I'm only one person involved, but my primary reason for choosing Theano over TensorFlow has to do with the ability to manipulate and reason symbolically about models/graphs.

In order to improve the performance and usability of PyMC, I believe we need to automate things at the graph level, and Theano is by far the most suitable for this--between the two, at least.

You can find some work along these lines in the Symbolic PyMC project (https://github.com/pymc-devs/symbolic-pymc); it contains symbolic work done in both Theano and TensorFlow.