I have seen at least one theory that Slytherin's original intended role fell apart over the centuries as it became the house for wizard racists. So ambitious people who weren't shitheads balked at being assigned there, and scions of shitty wizard nobility wanted in regardless of aptitude, and the hat takes that sort of thing into account
Of course in general it's just genuinely hard to write societies and subcultures made up entirely of ambitious, scheming backstabbers, as you can see with, like, drow, or Romulans
"Because if I go in there, they'll murder me in my sleep."