Identification of a part on a power line's lightning arrester
These devices appear to be for gallop arresting.  Long wire/cables (even structures) are subject to high-amplitude, low frequency vibrations that can cause self-destruction.  These vibrations occur in the presence of generally gentle winds of just the right velocity.

Factors such as the cable length, material, jacketing can affect its sensitivity to this affect.

My guess, in your case, was the engineering analysis indicated the lightning arresting cable was of a design that was subject to this gallop effect, while the power lines were not.