The reason for the difference is the same as most things in aviation - tradition. Fixed-wing started out using (and many still have this) a speaker in the ceiling or somewhere, and a hand-held mike. You can plug headphones in & listen, while still using the handheld mike. Helicopters don't really permit that - you need both hands free, thus the integrated helmet or headset. The U174 plug originated in the military, the primary users of helicopters. When fixed-wing pilots started using headsets, they needed something that would fit what was already installed, thus the separate plugs for mike & speakers. Too much installed base for both to change either.
I've built my own adapter, & it's pretty simple. I had a jack from somewhere in the military, long ago, & bought the fixed-wing plugs. The speaker plug is a standard 1/4" plug, used for guitar cords, etc, available at any electronics place like Radio Shack in the US. The mike plug is smaller, & I broke down & bought a plug from an avionics place for a few dollars. The cable came from Radio Shack, IIRC. It's 2-pair, or 4 conductors. Any cable with 4 or more wires in it will work. You just have to deal with the split/splices, but rubber tape works very will with this. Mine isn't elegant, but it works, or did the last time I used it, several years ago. Being a humble helicopter pilot, I can't afford to pay to fly airplanes very often.