I'm not philosophically opposed to supporting C extensions. The additional complexity just was not deemed to be warranted since the YouTube frontend doesn't use a lot of C extensions.
In principle it's possible to implement something like JyNI (http://jyni.org/) or CPyExt (https://morepypy.blogspot.de/2010/04/using-cpython-extension...) to bridge the CPython and Grumpy APIs. In practice, marshalling data across the interface can be very expensive.