The thing with Abraham and Giroud is that they each have something that the other person lacks. For instance, Abraham's hold-up play isn't quite as good as Giroud (yet, maybe) but Giroud doesn't have Abraham's pace and mobility.
Based on how Lampard wants us to play, having pace, mobility, agility etc is important and given he is also trying to build a team for the future, it's also clear why Abraham is preferred over Giroud. Giroud was never going to be part of our long term plan, especially where being first choice is concerned.