There are some really good reasons for the slide mounted de-cocking system, many of them are just not known.
1. It keeps the grip of the frame open for the hand.
2. Less chance of the hand interfering with the de-cocker.
3. Segmented firing pin safely drops the hammer all the way to the slide, not to half cock.
4. Keeps the sear to just using the sear notch on the hammer, often making for a better single action pull.