怎么 zěn-me mostly means how. Yet, in some context it is best translated with why. In English we have this too. How-come has a similar meaning to why.
你 | 怎 | 么 | 不 | 去 | ? |
nǐ | zěn | me | bù | qù | _ |
you | how | suffix | not | go | _ |
you | how?, howcome?, why? | no, not, after negative question: yes | go to, pass away, remove | ? | |
Why don't you go (= you should go)? |