You are comparing an encoder (x264) with a format (VP9). While there is a reference encoder for VP9 (libvpx). x264 has been one of the most heavily optimized encoders whose development was sponsored by many different organizations, whereas libvpx did not see the same kind of love and most of the work came from Google's employees (with some from outside, I'm sure).

There exist other VP9 encoders, but all for specialized purposes.