DID wasn't in the DSM until 1980.

"Strange Case of Dr. Jekyll and Mr. Hyde" was published in 1886, when multiple personalities was more commonly diagnosed.

In 1910, the term Schizophrenia was introduced. From that point on, cases of DID dramatically declined. It is suspected that individuals with DID were being diagnosed as Schizophrenic.

Sybil aired in 1976 and awareness of the phenomenon once again rose, as did the diagnosis of the disorder.