Why 50 ohms is used as characteristic impedance in PCB?

 The classic “magic” 50 Ω is a compromise between two extremes:

• Higher impedance (e.g. 75 Ω) gives lower attenuation per unit length but poorer power handling.

• Lower impedance (e.g. 25 Ω) handles more power but suffers higher dielectric losses.
  At around 50 Ω you get a sweet spot suitable for a wide range of RF power levels and frequencies.