John Gruber's contribution to the Mac's thirtieth anniversary is thoughtful and beautifully written. It's hard to overestimate how important these early details have been in the longevity of the platform, and in how influential it was over pretty much all modern computing paradigms.
So Copy was awarded the mnemonic ⌘C, and Cut the sort-of-mnemonic ⌘X, but Undo and Paste were assigned the semantically meaningless but ergonomically convenient shortcuts ⌘Z and ⌘V. Not only was the idea of Undo a novel invention, the Mac team found a shortcut to invoke it that was as easy to type as possible. And what is the most common thing to do after copying? Pasting. So what could be a better shortcut for Paste, ergonomically, than the key right next to the one for Copy? You remember these shortcuts not by letter, but by physical position.
Even these four commands’ order in the Edit menu corresponded to their shortcuts’ order on the keyboard: Undo, Cut, Copy, Paste — Z, X, C, V. Simply brilliant. Every one of these design decisions has persisted through today.