Because target priorities. Luckily, Protectors rarely have more firepower and ship numbers then Attackers.

It's still a good idea to equip protectors with Slugs IF you expect to fight missiles heavy fleets. Once you unlock Coordinators, you should switch Slugs to them.

Furthermore Protectors are usually more tanky than attackers so they have a better chance to reach the short range stage, at which slugs excel. 

GTI wrote:
and in the second table, the 600 hull weakness, does it mean large ships have the weakest hull?

Yes, but not by much. The higher hull weakness of medium and large hulls mostly just compensates for the fact that they get 2 or 4 defense modules per slot you fill. You can see your expected damage reduction if you toggle the "detailed stats" on the right of the ship design menu. So for example, a single Titanium Plating on a small, medium, and large hull give 55%, 49%, and 44% damage reduction respectively.

Thank you for the answers people!