Because 4 strokes require more parts to make and the parts are more heavy dutier than a 2stroks cos the 4 strokes have bigger engines, bigger parts = bigger money, more parts= bigger money.
well to start with 4 strokes are more complicated than 2 strokes.
They are more efficient in terms of mileage and also combustion which gives out lesser emissions. So they are more difficult to manufacture and thus the cost.