Past his prime Federer and the rest of the tour have not defended these two slams. Past his prime Federer is understandable due to age but the rest in their prime are not. They are not good enough to defend these two slams, considered the big two slams by many.
Well, Nadal and Del Potro didn't have the chance to defend Wimbledon 2008 and USO 2009 due to injury.
Federer, Nadal and Djokovic lost close finals (especially Fed and Nole) the year after they won the US Open, etc... Federer losing to Berdych in Wimbledon 2010 was the worst loss from a defending champion really.