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The origin of open wire feedline
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The following is excerpted from an old article about Parallel Transmission Lines, by L. B. Cebik.
"Parallel feeders go back to the beginnings of radio. By 1930, the "two-wire un-tuned feeder system" was a standard ARRL Handbook feature. The Jones Radio Handbook of 1937 provides a table of line losses showing the advantages of open-wire feeders (a 440-Ohm line in the table) over lower impedance twisted-pair feeders (p. 70). The use of 600-Ohm lines was fairly standard, using a spacing of about 6". "To reduce radiation from the feeders to a minimum, the two wires should not be more than 10 to 12 inches apart." (The Radio Amateur's Handbook, 7th Ed., ARRL, 1930, p. 162) Rarely did hams exceed the 6" spacing".
In 1930, parallel constructed lines were coupled or fed inductively to the final tank circuit of the transmitter.
Parallel feedline was the only choice for hams as well as commercial broadcasters in the 30's and 40's. Along came World War II and coaxial cable was developed for military use. Eventually, after the war was over, the surplus cable found its way into amateur use. Companies began to develop various types of coaxial cable for civilian use and as hams discovered the flexibility and greater options of using shielded line, manufacturers began to build radios to the 50 ohm standard we enjoy today.
Open wire feedline or ladder line still remained popular among many hams that ran higher power, demanded efficiency and could maintain the ideal installation. Even today, if you listen to any of the "Big Guys" on 40, 80, or 160M AM phone, they almost exclusively use open wire feeders to either a large doublet or loop antennas. It makes no sense to build a perfect antenna and then feed it with a 150' to 200' piece of coax and lose a fair percentage of power. Ladder Line is the only way to go!
Here's a link to an online calculator that you can play with to really see the difference. Use 1.5 as an SWR entry and then compare various coax types to 600 ohm open wire. There's no comparison!