It is likely based on the key position of other commonly used actions (cut, copy, paste) on the keyboard.
Likely it all boils down to placement on a QWERTY keyboard.
From there, X and V are just adjacent keys, for Cut and Paste. It's
simple to remember where they are, and you'll build up muscle memory
if they're close. (You'll do it even if they aren't, but if they're
adjacent, it helps.) Eventually, when Mac and Windows word processors
(and other applications) started offering Undo as a feature, putting
them together still makes a kind of conceptual sense: these are basic
document editing features which work the same way across even very
different applications ("Undo" is undo in Word, or Adobe Illustrator,
for example, even if the content types are wildly different).
Source: Why was 'Z' chosen for the CTRL+Z/CMD+Z shortcut?
It's also close to the control/command key, which makes it easy to use with one hand. Common actions (cut, copy, paste, undo) are used a lot, so it is important to have them close to the control/command key.