Well, in my guess, i would most likely say, the slugs are less noticable, and therefore, predators, dont spot them, in which even if they have a main predator, it would be quite difficult to spot. In which on the other hand for the hedgehogs, they are bigger and easier to find. And they have more meat on them then the slugs, so Predators would eat them before the slugs...
~educated guess :)