The letter c was already taken for change so they couldn't use "copy". They needed another word. They couldn't use w for write, because it already means word. They couldn't use p for put, because it was already used for paste. And so on. (http://www.gammon.com.au/smaug/vi.htm has a list of vi commands.) Eventually someone came up with a word, yank, that didn't start with a letter that was already being used, and that at least vaguely carried the right meaning. Plus for some people, they remember it as the last letter of copy. The fact that you can think of words that better mean "copylike behaviour" isn't really the point.