The FCC chose not to include zero-rating in the Open Internet Order, because of its ambiguity. They could imagine both good and bad approaches to zero rating (with the bad being to effectively circumvent net neutrality).

They've been consulting with the industry to try to figure out which aspects of zero rating are helpful and which are (or have the potential to be) harmful. That's what a good regulator should do.

The original Verge article describes all of this. Perhaps you'd care to read it.