To give a visual explanation...

In order to join shots together in the pre-digital era you either primarily used tape or glue (cement). Tape was used for the editing booth (easier to take off and on) and cement was used for joining the negative (much stronger and cleaner than tape). To show how they used cement to join film, here's this great video:

https://www.youtube.com/watch?v=7s_kB2_C668

You can imagine how nerve wrecking being a negative cutter could be. One bad splice and the shot is ruined.

Now why do we see this residue on scope films and not others? Well, let's look at the spaces between frames in a scope film frame:

Do you see how there's very little space? Now let's look at a sample of film frames from the 1.33 Academy Ratio (note: 1.66 and 1.85 aspect ratios are cropped versions of the academy aspect ratio)

There is MUCH more empty space between frames (the reason for this was so that films could keep their 1.33 aspect ratio when sound was added in the 20s). Because of this, the cement doesn't "spill" into the frame.

Make sense?