The big utility-scale solar plants tend to be on the ground in big fields. I think the preference for rooftop solar for individual houses comes mostly because it's convenient for suburban installations, as they don't usually want to give yard space and the panels would be in the shade anyways. Roofs aren't being used for anything useful, so that's where the panels go. In a rural setting there's less incentive to install on a roof, but you still might put them there anyways if that's where you get the best sun.

I suppose people might also be afraid of theft or vandalism if the panels are accessible to random passers-by without a ladder.