the slugs are less noticeable, and therefore, predators, don"t 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.
This is what I think. I hope this helps please let me know if you need anymore help.