I think it's just the freaky picture and the catchphrase he has going for himself. Slenderman may be overhyped too, but at least the creator didn't try to explain why he looks the way he does. The mystery is what makes him somewhat enjoyable. Same thing with the Pyramid Head, you don't have to know where he comes from. You don't need to know his bullshit tragic past, you don't need some idiotic reason for him to have the pyramid on his head. He's just there and it's terrifying.
"Mister Slenderman was a businessman, that's why he wears suits. One day, he was walking home from his job and he passed by a construction site. Suddenly, one of the beams from a crane flies loose and hits him in the head, knocking face clean off! And then, an octopus leaps from the sewer and sodomizes him in the middle of the street while he's bleeding all over. Then the tentacles grew out of his back! Because of his ghastly appearance, he ran from the world and went to hide in the woods, where he leaves notes everywhere, telling all outsiders to flee or else."
While that's all ridiculous in itself, it'll never be as ridiculous as Jeff The Killer's backstory. Where his hair is black because it was LIT ON FIRE. Honestly, when I read it, I thought it was a trollfic.
"Mister Slenderman was a businessman, that's why he wears suits. One day, he was walking home from his job and he passed by a construction site. Suddenly, one of the beams from a crane flies loose and hits him in the head, knocking face clean off! And then, an octopus leaps from the sewer and sodomizes him in the middle of the street while he's bleeding all over. Then the tentacles grew out of his back! Because of his ghastly appearance, he ran from the world and went to hide in the woods, where he leaves notes everywhere, telling all outsiders to flee or else."
While that's all ridiculous in itself, it'll never be as ridiculous as Jeff The Killer's backstory. Where his hair is black because it was LIT ON FIRE. Honestly, when I read it, I thought it was a trollfic.