The ratings weren't really that bad when ABC cancelled Dark Shadows, true they weren't as good as they had been during its peak, but they were still good. Maybe ABC had other reasons they wanted to cancel it.
I also read that the 1991 version had given NBC the best ratings that this time slot had had in quite some time. I'm not sure that this is true, but I did read it at the time the show was cancelled.
Misa