Why are Solar Panels mounted to bottom instead of sides?

I haven't seen any with side mounts, all seem to be underneath. I want to mount my new panel up to 1 1/4-1 1/2" for air flow. Most of the Z brackets are 5/8" to 1" Plus I need multiple mounting holes to the roof as mine is very thin and I will either use Drywall anchors which will mount flush or well nuts that have a lip raising the mount 3/16".