Because a gorilla walking through a basketball game with intentionally distracted viewers is nothing like a bag on the floor of a crime scene where law enforcement officers are specifically examining the scene for evidence.
Yeah, Law enforcement officers who were focused on locating evidence associated with a rifle crime.
Besides Oswald carrying his rifle to the crime scene in a brown paper bag, is there any other evidence in the history of the world where a brown paper bag was employed to carry a rifle to a murder and then left at the scene?
So overlooking a brown paper bag in a busy warehouse isn't very surprising.
JohnM