Morrison was a sick and depraved drug addict. His father is being kind.
"Let me tell you something. We need a two and a half hour movie about the Doors? Folks, no we don't. I can sum it up for you in five seconds, ok. I'm drunk. I'm nobody. I'm drunk. I'm famous. I'm drunk. I'm f'ing dead. There's the whole movie, ok!? Big fat dead guy in a bath tub, there's your title for you." - Dennis Leary
I'm sure it's easier on his family to try to think the best of Jim especially since he treated them very badly while he was alive and then made them very wealthy after dying in a bad way.