It goes by the number of downloads from PyPI. I guess that's lower for matplotlib, because installing it from PyPI usually means compiling from source, which is a pain for something as complex as matplotlib. Many people probably get it from a binary package manager, or from an all-in-one Python distribution like EPD.

Scipy is listed, and maybe matplotlib is considered a subset of scipy.

Now that scipy is supported, I'll start taking a look at python 3.