Julia was written in C++ in early stages if I remember correctly then Julia ;) The point here is that NumPy is heavily fine-tuned and will beat almost any other language implementation out there. The community is bigger and therefore there are much more free hands.
In Julia's case, there are maybe more "free hands" who can contribute to performance critical code. To contribute to `NumPy` you have to know how to program in C.
One of the important facts about OSS development is that the number of users doesn't scale linearly with the number of "free hands. There are less than 2 FTE's working on Python at any given time these days.