originally it was z x and c as just using the tools, but we ran into the issue of people constantly pressing the wrong one and getting upset that they screwed up the buttons, which, when there's strategy involved, is not fun. Being able to decide your tool, see which tool is selected for verification, and then using the tool worked a lot better for the playtesters I got.
As for the arrow keys, i don't think i've ever heard of a keyboard without them, so I never thought to not use them. it certainly would be clunky to use zxc and wasd, but when you're using arrow keys, they're on the opposite side of the keyboard, so if I went with wasd, I would also need to swap out the tool buttons.