Python 3 is a more modern, better language, that's why. Hopefully, grumpy will support it soon enough, though it's understandable that they started with 2.7 to support Google's huge older codebase.
But the bigger problem other than lack of widescale user adoption is that the primary reason for Python3 (unicode) was botched. Go got this right, everything is a byte string and the assumed encoding is UTF8.