...and also it is misleading because there are not 'multiple personalities'. Rather there is one personality that is not integrated. Dissociation is a mental defense mechanism that prevents the integration of traumatic experience. When trauma is experienced by a young child who has not yet formed a stable sense of self, and that child does not have external support to process the traumatic experience, the various aspects of the trauma - mental, physical, emotional, and sensory impressions - cannot be processed. If the trauma recurs over time these dissociated experiences of the trauma become more developed states of being. They cannot be integrated into the child's psyche, so they remain split off and become more discrete over time. Eventually these dissociated states can develop a separate sense of identity, thus the name.

Dissociated aspects that are 'called upon' more frequently become more complex dissociated states (identities) while other dissociated aspects may have a very limited function or experience and are perhaps more like holders of a single dissociated experience as opposed to a fully developed alter.
So the name change reflects a better understanding of what it actually is and how and why it develops. It isn't that people have 'many personalities' existing within them. It is more like that because of critical experiences and conditions in early childhood their personality was unable to develop in the usual way. The child was unable to integrate his or her overwhelming experiences into one functioning identity and instead the dissociated self-states developed separately.