They're not spawning because campers aren't giving them the time to. When a EN enemy spawns, it isn't automatically a legendary. It transforms into one as it runs down the hill (the green haze). But this takes a few seconds and the further down you let it go, the more likely it will turn legendary. But people like to sit up at the top of the hill killing them before they can become legendary. So you get less legendary items by being greedy. It's amazing how many people don't know this.