Actually, there is a good reason for 75 ohm coax. If you plot cable loss versus impedance, you will find that minimum loss occurs at 75 ohms. When you've got miles and miles of coax strung everywhere, as cable TV companies do, you want to minimize loss to reduce the number of booster amplifiers needed. In addition, since 75 ohm coax has lower capacitance per unit length, it has better high frequency response than 50 ohm cables. Best power handling ability occurs at 30 ohms impedance. I don't know that any industry uses a 30 ohm system impedance, but - 50 ohms is a good compromise between minimum loss at 75 ohms and best power handling at 30 ohms. (All this is the best I remember it, there might be a bit more to it that I have forgotten over the years.)
Jim Allyn - N7JA