Because
A. The bottle found at the very step that PM steps down too, by the west wall is NOT A COKE
B. The Dr.Pepper was Oswalds favorite drink, and it’s much easier to get from the ground floor machine than to have to climb up 18 steps and go thru 2 doors and enter into lunchroom and have to go back down 18 steps again
C, the COCA COLA white cursive lettering is MINIMAL white area on the bottle as opposed to the SOLID white oval shaped Dr.Pepper bottle. This the white label half exposed in Weigman film, above PMs hand holding at the base is the more probable
D. The Allen photo of Dr. Pepper bottle on the steps of TSBD in the slot recess on the very step by PM is SOLID white oval logo WITHOUT the diagonal dark bar which makes it NOT the bottle seen in the hand of a cop in the photo Mr. Ford posted earlier here in this thread.