John Gruber wrote about Apple’s attention to details.

Now consider keyboard shortcuts. The basic idea behind keyboard shortcuts on the Mac was and remains that you hold down the Command key, then press a letter key. And the letter keys should, ideally, correspond mnemonically to the menu command they represent — and for common operations, the shortcuts should be standard system-wide, across all applications.1 So: ⌘S for Save, ⌘P for Print, ⌘Q for Quit. But then what about Select All? ⌘S was already taken, so: ⌘A, emphasizing the All rather than the Select. ⌘D for Duplicate, ⌘B/I/U for Bold/Italic/Underline, respectively. And so forth.

They ran into some problems with other shortcuts:

⌘U could not be used for both Underline and Undo; likewise for ⌘C for Cut and Copy. And ⌘P could not be used for Paste because it was already used by Print.

The solution is still used up to today:

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.

Microsoft followed suit and used the Ctrl key instead of the ⌘ key.