Unlike most ArcPy modules, ArcPy Mapping (arcpy.mp) had to be rewritten from the ground up since the UI and structure of ArcGIS Pro is completely different than ArcMap. They will be playing catch up for quite some time, which is why it is important for people to open enhancement requests and create ArcGIS Ideas. There is a lot to port over from ArcMap to ArcGIS Pro, and without user feedback, Esri is left to either do what is easiest first or guess at what users want.