Why is a just a rather odd wh -word. Its distribution is very limited -- it can only have the word reason as its antecedent, and since it's never the subject it's always deletable. Consequently it behaves strangely, as you and others point out.
Why have a letter in a word when it’s silent in pronunciation, like the b in debt? Can anyone please clarify my uncertainty here?
Possibly relevant to why /z/ won: if I try to pronounce axis with a /z/ instead of an /s/, it sounds hardly at all different. Someone with a better understanding of phonetics can probably explain why the contrast is so small in that context.