Pretty sure it's the consumer. Most bikers commuting on road bikes don't need an e-bike, while most people wanting to try e-bike commuting are non e-bike cyclists. Thus step through flat bar e-bikes are made and that's what you see mostly for rental in cities. Smart commuters would run the road e-bike since it's more efficient, more agile, and probably more fun.
While most pre e-bike commuters are happy with their setup currently, so they haven't gotten an e-bike.