Asking why with "zenme"

怎么 before a whole clause asks "how come" — with surprise or complaint baked in.

Subj. + 怎么 + Verb Phrase ?

How it works

你怎么没来? isn't a neutral request for reasons; it means "how come you didn't show up?".

为什么 is the neutral "why". Use it when you genuinely just want the reason and don't want to sound annoyed.

This is a different job from 怎么 + verb, which asks about method.