I know why the coiled cord is used by helicopter pilots, but I have no clue as to why straight cords are used by plank drivers. I think it's probably tradition. Planks originally had overhead speakers and a handheld microphone, no headsets. When they started using headsets, there was already a separate mike jack, and it was easy to just add a speaker jack, using two separate plugs from the headset. That's still the way most fixed-wing are outfitted. It's inefficient and clumsy, but that's what they have. Helicopters from the outset used headsets, because it's not easy or even always possible to grab a mike whenever you need to talk on the radio, because it takes both hands for the controls. Helicopters have always, as far as I know, had a single headset jack for each crewmember, and the headsets have a single plug. If you spend a lot of time with your head out the window, looking places other than straight ahead, a coiled cord works much better, and doesn't get tangled in the collective.