The history of 50 Ohm impedance dates back to the late 1920s/early 1930s, when air-filled coaxial cables were designed for radio transmitters. Engineers needed to balance the cable’s ability to transfer power while minimizing attenuation and voltage loss. It turns out that finding a single impedance to balance these objectives was impossible. However, for air-filled coaxial cables operating below the cutoff of the TE11 mode, the power transfer was found to be maximized at approximately 30 Ohms. Selecting 50 Ohm impedance resulted in a compromise between minimum loss, maximum power transfer, and maximum voltage, which is why it’s widely used in RF/high-speed PCB design.
https://resources.altium.com/p/mysterious-50-ohm-impedance-where-it-came-and-why-we-use-it