The wedges are a low cost casting, based on the cross section of the break. Given that they only experience force in compression, it is not a bad choice, just not a great one.
Cinelli1RClamp on Flickr
Although there is a radius from the ramped feature to the curved surface, there is not a more gradual thinning of the curved surface from the ramp feature. This is due to the feature of the opening of the stem where the ramp fits. So the unsupported surface is a long lever that does not have reinforcement. It won't bend like a diving board and breaks.