I don't know of any big cities that have banned cars from anything but small areas in their centres. I know that in my city, the centre of which is pedestrianised, nevertheless has many car parks, including two large park-and-ride facilities with large car parks that could have solar panels installed.

The reason to build them over car parks is because the ones being considered are surface-level, so any building work is cheaper and easier. And it also provides a benefit to users in the form of shade.

Ultimately we should indeed aim to cover rooves with solar panels, but let's focus on the lowest-hanging fruit.