[acdha]

Separate keys have challenges: in addition to the extra cost, you need to find physical space and train people to look for and use them. The original designers wanted to make it efficient for people who were already typing and as you might have noticed those keys are all close together and convenient for one-handed use:

> Why the Z X C V keys? — They were close on the keyboard. We did X because it was a cross out (CUT). We did V because it pointed down like this [he makes a ‘V’ shape with his hands], and you were inserting; it was like an upside-down caret (PASTE). And Z was the closest one, because we figured you’d UNDO a lot. And C for COPY — that was easy.

https://www.youtube.com/watch?v=OW-atKrg0T4 via http://morrick.me/archives/8432