My theory is that while technically anyone can become a Guardian, Ghosts are very picky. This is due to the early days of their mission, when the need for quantity outweighed the desire for quality. Early Guardians, possessing their personalities, sentience, and free will, could have morals, relationships, and pasts that'd endanger them and their duties. The dead, though, have no such complications. Whoever they were before died with them, as well as their pasts. Dead Guardians could be shaped into what the Light needed much easier than living people.