Because connecting dev funds to social engagement is a pretty poor tool, imo. It makes those most likely to be great at dev work and most likely to not be wasting time on social, the least likely to actually receive any funding. In addition, social media is a network of very shallow interactions and short term engagement. Dev funding and donations need a longer term approach. It makes sense to outsource to a group who out in real time and long term effort to finding good teams, disruptive projects, and underappreciated devs. All things that would likely get lost if left entirely to zaps.