There was actually a good article in The Atlantic recently answering this very question because of a real incident: https://www.theatlantic.com/politics/archive/2018/05/is-this...

The tl;dr is, because an implicit code only works as long as no one actually uses it. As soon as an unspoken CoC is actually counted on to do something, people realize they didn't have the same expectations about what it meant, and so everyone on every side ends up angry and feeling they were treated unfairly.