I think it was fairly standard for soaps to do the one-take thing. Soaps weren't thought of then as they are now and didn't get the respect they should have from the networks. After all, in its time,
Dark Shadows was thought of as little more than a kids' show.
The videotape process was also still very new and fairly expensive at the time as well.
And even during this time, soaps like
Edge of Night (where Joel Crothers ended his career) were still done
live most days, barring preemptions--which led to some major flubs; the biggest I remember from that show was an actor's toupee coming off during a fight scene!