Because it causes engineering overhead that perhaps no one there wants to do. And when reddit gets its IPO it will no longer give a hoot and an half about the "average redditor" because profit suddenly becomes the only thing that matters to shareholderds.
What I am saying is that the visibility that coins make is prolly not inline with the people that want to use reddit to push products, shows, and ideologies for profit.
It is just about control.
That is a good point.