Doctor Who and the Brain of Morbius

Why do so many spaceships crashland on Karn, a bleak, lonely and seemingly deserted planet?

Are they doomed by the mysterious powers of the strange, black-robed Sisterhood, jealously guarding their secret of eternal life? Or does the mad Dr Solon, for some evil purpose of his own, need the bodies of the victims? And more especially, the body of DOCTOR WHO ...