You are much less likely to reinvent the wheel if you can add your one critical niche feature / bugfix to an existing library. In python, learning C and C build systems and python's C API are gigantic barriers to doing that.
More importantly, if every fast data manipulation needs to be written in C, a few of them can be profitably shared, but you need more than a few of them. Pretty soon you wind up with a giant dumping ground of undiscoverable API bloat. See: pandas.
