Well it's sort of complicated because of the whole River's timeline going the other way thing.
From her point of view, The Doctor has always known who she was, and was probably told that there would come a point where River would 'reveal' herself to The Doctor, Rory, and Amy, and then (FROM HER POINT OF VIEW) every time after that they wouldn't know who she was.
So she didn't tell them sooner, because sooner for Rory, Amy, The Doctor and the viewer is later for River and she's already done it all anyway.