Weight and structural integrity (and cost certainly also)
Not sure why on the right side… but I think some of it must have to do with having the pilot in first, possibly so you have weight ahead of the CG and don’t load passengers into the back first (risking a tail strike while getting aboard)?
As to emergency, it is a lot easier to push out the windows that operating that door mechanism, the windows are thin plexiglass held in place by old chewing gum and wishful thinking 