According to this site- http://hunter.apana.org.au/~gallae/pantheon/social/cultofcybele.htm

various factors contributed to the fall of the Isis cult, one major one being the efforts of the early Christians to destroy the cult and get rid of the Gallae, or transgendendered priests. The Gallae apparently arrived later during Rome's worship of Cybele. The Christians who would eventually persecute the Gallae weren't the only ones who didn't like them-Romans didn't like their overly frenzied rites. Perhaps the worship was a bit much?

As far as Domitian is concerned, perhaps his own personal feelings for the cult of Isis as well as the above may have contributed to his decision in banning Cybele's cult?