Because in general, the Warrior is supposed to use brute force, and this is why she wields weapons and shields, and she gets 2 weapons or 1 weapon + 1 shield. Paladin is supposed to be a defender and a leader, this is why she gets more regalia slots and a weapon + shield. The mage is supposed to be the master of the arcane, and this is why she gets 3 whole artifact slots.
I also consider artifacts to be one of the best items one can equip. The other classes may still make due with their slots, though
- and to good effect!