A spacecraft needs multiple receiving stations to prevent its radio signals from being blocked by Earth. Wikipedia has a map of the ground stations used for the Mercury missions.
The Right Stuff describes Mercury Atlas 6 being handed off between different CAPCOMs as the capsule flew across different parts of the planet. From listening to Apollo tapes, I know they centralized communications with the spacecraft in Houston. How was NASA able to do that for Apollo and what prevented them from doing it for Mercury?