Why is the electrical length of a coaxial cable longer than its physical length?

An 'electrical length' represents the portion of a wavelength that a physical segment of line acts like. Because the dielectric material inside a coaxial cable slows down the electromagnetic wave compared to its speed in air or a vacuum, the wave doesn't travel as far during one cycle. Therefore, a physical foot of coax contains more 'electrical degrees' of a wave than a physical foot of air. Consequently, to get a specific electrical length (like a quarter-wavelength), you need a physically shorter piece of coax than you would in free space. Distractors involving the skin effect are incorrect because skin effect dictates that RF travels on the outer surface of conductors, which affects resistance, not propagation speed.