Plenty of rooftop space for solar.

Yes, but not plenty of support or access to make it practical.

its _far_ cheaper to retrofit carparks than it is roofs. generally there is nothing under a carpark apart from tarmac. Unlike a roof which was designed for a specific weather loading (be it wind, rain or snow)

You also need to attach the panels to the roof, its generally simpler to bolt into tarmac compared to bonding to a weather proof seal.