harry potter - Why do less-bright students end up in Slytherin?

The main trait of Slytherin house is ambition and cunning.

Malfoy fits the bill. Snape fits the bill. Harry Potter has both, to an extent.

But how can someone argue that Crabbe, Goyle or Millicent (or Pansy if Hermione is to be believed, though she's a bit biased) are intelligent enough to be ambitious (never mind cunning)?