Both are iconic because they're "easy" to pull off when clowning around with your friends.
No one wants to get slammed on their back (Rock Bottom), or head (Tombstone), or hands/stomach (Pedigree) or go in the air (leg drop) when f'n around with friends, but the Stunner and RKO is something you can pull off quick and easy for a laugh. A lot of moves require strength and balance, but the stunner and RKO can be done by the everyday man. Thus their popularity.
RKO took a lot of the Diamond Cutter's shine though.