I think it is partly that. Brutal Machiavellians (B.M.s) who've made it to the top won't believe that anyone EXCEPT other B.M.s can be effective managers, so they're the ones they promote. And so the situation perpetuates itself.
This is why you tend to find that companies are either dominated by B.M.s or have very few of them - it's rarely half-and-half. (I also work in media but I'm sure it's true in other industries as well.)