What's really neat linguistically is that "helicopter" isn't a compound of "heli" and "copter," but rather "helico" (as in helix, helical) and "pter" (as in pterodactyl).
"Rebracketing" is when this happens (i.e., the split in the word is moved in colloquial language).
Not 100% sure on this, but it may be due to 'helo' being more audibly distinct than 'heli' over a shitty garbled radio?
It could also be from different regional/national accents of English pronouncing helicopter as basically hee-lo-copter, sort of like how there are different pronunciations of Uranus or nuclear?
Well, to expand on that theory, 'helo' would have originated almost entirely with the radio tech of 50s and 60s military.
A whole lot of lingo basically makes sense if you understand its origin, but when the term keeps being used for decades and decades, its removed from that context and doesn't seem to make much sense in a modern context.
Heliskiing would be a lot more interesting if it was a fuel-efficient way to return the helicopter to the airport down in the valley after flying something up to the mountain.