Because of the pdef and bonuses of the heavy armor.
where i can see these bonuses?
also does anyone know what are the effects from shield options (attack and defense type)? i mean you can make it attack or defense but i can't see no where what stat it gives. Also does it work only with heavy set or you can use it with light set as well, in term to get the bonuses.
where i can see these bonuses?
On the heavy set itself.
Light is used against mages (pdef is meaningless versus magic, after all...), but most times Tyrrs are faced with fighter opponents, that's why heavy armor is the primary choice.