Why are Solar Panels mounted to bottom instead of sides?

I have used simple door hinges bolted to botom one side of the panel and other to the roof using stainles steel bolts..
squeezed some silicone into the roof holes and pushed the bolts thru roof,,fastened,tightened the nut on the inside..
Used L shaped custom made aluminum brackets on the uper side of panel to make it tiltable,,drilled holes in the L brackets for bolts to hold panel in raised position..
Holds great and no leaks yet..

I think there are some how to do posts on this site with pictures..