Because left side atrium and ventricula receive too much blood (blood that normally returns plus Blood that pass through the arterious doctus), and so they respond with dilatation and hypertrophy. But i think Is not True that right ventricula Is normal: if the quota of Blood that pass through the ductus is important, causes and incremention in pression in lung arteries and, so, hypertrophy of right ventricula (in severe cases, eisenmerger syndrom).

Blood flows from aorta to pulmonary artery creating left to right shunt resulting in left atrium and ventricular overload