Usually it's circumstantial. So if you live far away and an interview just isn't feasible to do in person, they'll offer a Skype one, although with reluctance. I lived a 6 hour drive away, and I had to come down for a Friday, missing both Thursday and Friday at college for travelling and the interview. It's not really your choice, although if you really can't do it in person, provide them with a good enough reason to give you a skype interview.