London Train Stations North of the River ?

Does anybody know why so many of the London overground stations are located just north of the river Thames ? the trains come from the south, cross a bridge and then stop. I would have thought it would have been much cheaper to locate the stations south of the river and have a footbridge to take people over the river. The following stations are all just north of the river : Cannon Street, Blackfriars, Charing Cross and Victoria