I'm not sure at what point cakes became the norm but in his latest series "Heston's Great British Food", British chef Heston Blumenthal explains that throughout the 17th century it was actually pies that were served at weddings.

These pies would be huge and contain a number of secret fillings that wouldn't be revealed until the pie was cut and served to guests.

I'd imagine that the cake became more popular as sugar became cheaper and more readily available to the masses, whereas pies had even be known to contain rotting and undesirable flesh - affordable for even the lower echelons of society.

EDIT: Grammar.