It is because the standard coax impedance in RF&MW is 50Ω, and also virtually all test&measurement instruments use this refference impedance, also many devices and circuits are matched to 50Ω.
50Ω was chosen for coax as a tradeoff for both TX and RX operation. By optimizing the coax size, 2 optimum values were obtained:
- approx 37 ohms for maximum power handling;
- aprox 76 ohms for minimum attenuation.
50Ω is a tradeof between the two.
Where only reception is needed, or low power, 75Ω is used - TV, and old analog telecom lines.