Why are Solar Panels mounted to bottom instead of sides?

DAS26miles wrote:
My roof is fiberglass only 1/16" thick and 1/8" luan plywood and 3" of styrofoam. Not much to hold the screws.

Not pretty maybe, but on the truck camper I didn't have good places to attach brackets either, so I used 8 ft 2x4s across the roof with brackets to hold them at their ends to the solid top frame at the edges of the roof each side. I then attached the panels to the 2x4s. You could use metal pieces across the roof instead of wood same idea.