I agree that there should be one and only one protocol, and ADS-B seems to be best indicated. However, being certified and all that, ADS-B equipment must be expensive. I think authorities are hesitating, not to say reluctant, to impose yet another costly bit of equipment, after 8,33 radios and mode-S transponders. It may all be very well for owners/operators of upper class planes, say a PC-12 or a Beech Baron; but someone who bought a second-hand ultralight for perhaps 20.000 euros and already had to spend 5.000 on avionics upgrades will not be looking forward to more investment in that corner.
Let us also not forget that, if once a mode-S transponder is present, ADS-B in and out are relatively easy to implement, indeed I think many mode-S transponders already have an option for ADS-B out. The crux comes with displaying the data from ADS-B in.