I am pretty sure they are focused on weed because it's an emerging market whereas the pharma delivery(which home delivery does exist) is an entrenched market with well established incumbents. also, weed doesn't require a prescription(past the doc's initial rec) like a script for pharmaceuticals does.

The main reason I think is that pharmacies are a major part of drugstores and are easily accessible whereas municipalities are free to say "I don't want any dispensaries here" so there is a vacuum of availability for many people. driving to the corner pharmacy or taking a short subway to the pharmacy is different than making a cross town trip to get the medication you need.