Why are implantable defibrillators placed on the left side of the chest instead of the right side?

Most in left side: Because the electricity to defibrillate flows between the electrode that is positioned in the right ventricular floor and the defibrilator generator (that is what closes the circuit). Most of heart is in between those two electrical poles. Still, they can be put in the right side of chest and the circuit is closed by the generator or by a coil of the RV electrode (in the superior vena cava IVC)