How does monopsony power evolve? What factors might aid its continued presence?

Monopsony may arise because a person within a given locality has only a relatively few choices of where to work. However, if the individual was willing to move, or the number of buyers of services potentially is enlarged, then the role of a monopsony is diminished. So monopsony power is more likely to be present where movement and search costs impede workers from seeking employment in different locales.