Cecil Moore, W5DXP wrote:
"I vaguely remember something about efficiency versus power handling
capability being the difference in the 75 ohm standard and the 50 ohm
standard."

That seens exactly right. It`s the reason you would use 75-ohm Zo cable
for TV distribution at low-power where you want to minimize loss but
there is no danger of too much voltage flashing over the cable.

Terman in his 1955 edition on page 106 says:
"---in an air-insulated coaxial line of given outer radius b, Q will be
maximum when the inner conductor has a size such thet b/a = 3.6.
(b=inner radius of outer conductor in concentric line, and a=outer
radius of inner conductor in concentric line) corresponding to Zo = 77
ohms. These are also the proportions for minimum power loss----."
However the maximum power that can be transmitted without exceeding a
given voltage gradient occurs when b/a 1.65, giving Zo = 30 ohms."

So for minimum loss you would want Zo of about 75 ohms and for maximum
power capability you would want 30 ohms. I suspect Zo=50 ohms is a
compromise between power handling capability and reasonable loss.

Best regards, Richard Harrison, KB5WZI