Because in this neighborhood of the superimposed wave function that comprises the multiverse that didn't happen. There are also an infinite number of neighborh

Because in this neighborhood of the superimposed wave function that comprises the multiverse that didn't happen. There are also an infinite number of neighborhoods where the ability to ask that question doesn't exist because of the event. To ask why it didn't happen in this particular neighborhood is to ask why the lack of that event is possible. In a Q.M. multiverse "why" questions get real tricky.