Why was the Electra uniquely vulnerable to whirl-mode flutter?
Flutter is a natural frequency resonance phenomenon.  Something is oscillating, and its oscillation, at some point, starts to match the natural frequency of the structure it's attached to, and a mutual input/feedback cycle starts that stores and returns the energy in such a way that it magnifies the initial input and away you go.

The outer wing was the saxophone... the engine nacelle was the reed.

The other airplanes?  Well, when you get down to it, fortunately they were just out of tune you might say, so they weren't affected.