There is no why! There would only be a why if you completely control the switching or dissociation. It is a mental illness.

Would someone ask a schizophrenic, "Why are you schizophrenic?"?

In my case of dissociation, people do not see a difference in my personalities so no one would ask why. This makes me getting a diagnosis extremely hard and, to date, I have gotten no help.

I have been trying to educate people about myself by telling them with me dissociation is more about amnesia/memory problems than extreme differences in the personalities.