PCB Impedence - Why 50 Ohms?

Can anyone tell me why do we use 50 Ohms Impedance in PCB signal layer and not 60 or 70 Ohms for non differential tracks?

50 Ohm is a industry standard for most of RF and Microwave design and measurements.
Because 50 Ohm is a compromise between 33 Ohm and 75 Ohm that theoritically gives you maximum power handling capability and minimum loss respectively.
So, 50 Ohm is a optimum point between these two..

Thanks for the input