Hacker News Reader

Probably the same reason (or at least somewhat related to) that Google App Engine only supports python 2.7. (https://cloud.google.com/appengine/docs/python/)

Big companies are moving slow, even Google.

Python 2 is used internally at Google, and TensorFlow is, I believe, an open sourcing and nicer packaging of something that has already existed internally for awhile.

because the low risk, high-audience strategy, when you're launching a new product, is to follow the path of least resistance, rather than being dogmatic. Maximum audience.