Why do only Roman Catholic priests perform exorcism's?

They don't. Many church leaders have performed exorcisms although they are not as you would see in the cinema - nor are they exclusively Roman Catholic. In the Anglican Church, for instance, most dioceses have a priest whose role it is to counsel, advise, comfort and so on those who have had rather disturbing experiences through dabbling with the occult. Coloquially they may be called exorcists, and they do often visit homes and other places offering prayer and support, but they are not always as the cinema or books portray - the image of an elderly Roman catholic priest splashing holy water about with a bell, book and candle. Nevertheless, exorcisms do actually occur across the Christian church as more and more people get involved in dangerous occultic practices and become spiritually and psychologically damaged as a result.