that line used to be: Film at 11.

and the very real reason was that, before 1975 or so, they shot location stories on 16mm film. it had to be processed before we broadcast the images, so it wasn't ready for the 6 o'clock news. the TV studio had a big stinky wet chemical developing machine in a back room... by 1975, they had portable U-matic or Betamax tape, with luggable battery packs, and the developing machine was left to rust.