There was a period in American Horror, from the 70’s onward, that spawned a certain type of film. Ostensibly it was seen as a reaction to the Vietnam War, not a politically motivated action, but a subconscious one. The violence that was shown on television at the time permeated into the violence you would see on screen. With the leaps being made in special effects (What better frame of reference than some news footage on TV) it meant that filmmakers could push the boundaries of what was seen before. Indeed, filmmaking itself was getting cheap and mobile enough that it was easier for a Director with no budget to go out and shoot something.
It’s this perfect storm of influences that led to Wes Craven’s The Last House On The Left.