Because we'll never go to an all GPS system. We still will have airspace where ADS-B out won't apply where Mode 3A is still relevant. Still will need an ELT for SAR purposes, which is completely separate from ADS-B
On the military side, several on here have expressed concern for the vulnerability aspect of using ADS-B. Even if the military gets all their aircraft equipped with a secure version by 2020, they'll still hold on to their traditional transponder boxes for Modes 1-5.