Not sure but they make sense in proximity to ctrl (at least on a US layout).
and further, the characters make sense.
X looks like scissors in open position, c for copy, v as a carrot pointing where you want stuff...z is in the shape of putting somthing back from a slice of memory if you stretch your imagination.
Why does it matter ?
Maybe it doesn't, but it's interesting history that a decision made can withstand the tests of time as much as this.