Because neither one can polka.
I found something similar to my answer, after I had derived it, on Cecil Adam’s “Straight Dope” page.
But there, they referred to quills rather than plumes. I like to think that the image of plumes of ink very aptly describes a raven.
Of course, writing desks with inkwells and quill pens are a bit out of fashion. On the other hand, plume is still the French word for pen.