Takes more infrastructure to set panels up up high over a parking lot full of drivers than in an empty field so that delays the solar transition a bit - you likely want as few new posts/pylons in a parking lot as possible but the whole rig still has to survive some idiot in a F-650 plowing into a post without toppling and crushing a bunch of cars and shoppers. I suspect high winds might be more of an issue but I'm not a civil engineer. Throwing them in a field is a bit quicker.
Maintenance is more of an issue too - the elevation adds some accessibility challenges. Plus, do you close the parking lot whenever people are working up there to minimize the liability/risk someone gets brained by a wrench? Parking lot operators are skittish enough of falling tree limbs that they often remove any trees from the property.
TBH I'm happy with any new solar and it's certainly an improvement over parking lots as they stand now, plus it puts them close to the places using them which is great. But I also think expedience has a real value at this transition point.