I always thought it was because the impedance of basic antennas are
around 50 and 75 ohms. Here is another story I came across.

"In the United States, the predominant impedance for coaxial
transmission lines and connectors is 50 ohms. The theoretical impedance
for minimum attenuation is 77.5 ohm and for maximum power transfer it
is 30 ohms. The average of these two impedances is 53.75 ohm or rounded
off to 50.

Therefore, 50 ohm is a compromise between minimum attenuation and
maximum power transfer in a coaxial transmission line, and that is why
it was selected. There are connectors available with other impedances,
the next most popular impedance being 75 ohm (approximate minimum
attenuation performance) that is in fairly wide use internationally and
in long line communication systems."

75 ohm cabling is also often used for video applications.

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Telamon
Ventura, California