In a one way ticket, the airline has to find another one to fill up the seat on the return sector. if no one taking it, an empty seat means less profit or some loss.
It is because the flight company want the costumers to buy return ticket which normally cheaper than one way ticket. but it just a marketing strategy to make them gain more money from the costumer.
because you go to the place you can play and do other things.but you return ticket just go black.so the one way flight is expansive than return ticket .(just my idea and my english is bad,i hope you can know my answer.)