Around here where I live and work when I encounter someone that doesnt believe in DID its because if they did they would have to believe there are such horrors in the world that caused the DID...

Example one of my neighbors does not believe in DID. her reasoning is that if she believed in it, to her it would cause her emotional pain knowing I had been hurt in such horrible ways beyond her comprehension. She cares a lot for me and like a mother who is bonded to their child, it hurts the mother to know her child hurts. it would hurt her to know the pain I was in.

there are lots of general reasons why a person doesnt believe in this or that mental disorder but only those in your life can answer why they dont believe.

my suggestion - maybe you can do what I do.. when someone tells me they dont believe in something (no matter what it is.. angels, higher beings, mental disorders, physical health diseases.... what ever) I ask them why. that way I know the answer from their perspective.