Sometime, in the late 1960s, the horror genre changed; it became mainstream, with big-name stars and bigger budgets. It was no longer more or less the main province of genre fans and stars like Vincent Price, Boris Karloff, or Lon Chaney, Jr., or established genre directors like Terence Fisher or Freddie Francis.
In this post from my personal blog, I date this change to Roman Polanski's smash hit, Rosemary's Baby, which debuted in 1968. Rosemary's Baby had mainstream production values, mainstream actors, and a director whose talents were far above those of the majority of horror filmmakers of the time. It was quickly followed by other mainstream horror films from big-name, mainstream directors, such as William Friedkin's The Exorcist and Richard Donner's The Omen.