It’s reputation

But those two scenes have more graphic bloodshed and violence than all the Westerns that came before them combined. And all in sloowwww motion so we get a good long look at it, too. No Western before or since ever captured its impact, even the other ones Peckinpah directed.

I agree with Whynotwriteme, prior Westerns usually just had the shooting victim clutch at his belly, gasp and fall down

Pekinpah stylized the violence, dragged it out and included actual looking injuries