It was because CBS bought "The Secret Storm" from American Home Products in the late '60s and proceeded to destroy practically every reason people watched the show: the Ames family (the core family) was practically written out, some far-fetched plots meant to resemble "Dark Shadows" (its ABC competition at one point) were written in, and the show's time slot kept shifting. In 1968 it moved from its traditional 4 PM to 3 (ET), where most soap viewers were committed to either "General Hospital" or "Another World." In the fall of '72 it moved to 3:30, against "One Life To Live." Finally, in the spring of '73, it went back to 4, and sometime over the next few months CBS sold it back to American Home Products, the Ameses were brought back, and "Match Game" became "Secret Storm"'s lead-in come July 1973. It didn't matter; the audience was gone, and an attempt to keep it going in syndication fell through before even one episode was taped.
