In the last half of 1970, the ratings dropped badly, which led to it's cancellation.
Second, i belived if the show would have continued during ABC golden age of daytime in the late 70's and early 80's. Dark Shadows would have become General Hospital and the older cast would have been replace with hip new younger people and have Adventure stories about artifacts. I think the original appeal would have been lost.
Speaking of General Hospital up until recently I was also a big GH fan but stopped watching because of the loathsome backstage politics and the mob Sonny Corinthos hour BUT... for years GH had a wonderful dark romance between Laura Spencer and Stefan Cassadine worthy of the best of the Dark Shadows gothic romances.