How come you only get the sonic damage on a critical with a thundering weapon, but shocking, etc. get to add it for any successful strike?
1) Sonic damage is almost never found as an energy resistance type of a monster. Therefore, sonic damage on a weapon has to be a bit more scarce to equate the value of a flaming weapon and a thundering weapon. Otherwise, why have a flaming weapon when you can get a sonic weapon for the same price?
2) Unlike other elemental weapons, thundering also inflicts a status effect (deafness).