Who knows. I remember I first started riding only the KHE & Poverty coasters were on Danscomp. I remember the Stolen coaster & Alienation ones being on there too but not till later. Besides flatlanders, I can't think of many OG freecoaster riders besides Seth Kimbrough, Ian Schwartz, Bruce Crisman & Karl Poynter.
That being said is why I think the coaster trend began to blow up, and I'll explain shortly. Not many companies were still making freecoasters (Primo, Odyssey, and I'm sure more all had old freecoasters but were no longer making them, besides the Odyssey but idk when that stopped) so they weren't that widely known, especially given the number of people you would see in videos riding one. It was rare.
I know Seth Kimbrough was supposed to get a signature coaster through Shadow years ago before they made one, so what I think happened is when he approached Shadow asking for one, and rumors surfaced that they would be making one, other team members on other companies, hypothetically let's say Demolition for instance, were like "Hey Demolition, I heard Seth might be getting a coaster through Shadow soon.. Could you make one for us to ride too? I don't really like how the KHE ones look, oh and maybe tweak a few things to make them more to our liking like slack adjustment, etc?" and Demolition was like sure! and rumors then started that they were releasing one, and other companies were like "Shit, we better hop on this coaster thing too so we're not left behind!" so they started making their own too, and to advertise this, they had their heavy hitters ride them to test them and more importantly, show off to their fans "Hey, your favorite rider rides a coaster & look what he can do! Maybe you should get one too!"
So more & more people converted to riding coasters because their sponsor was testing them & wanted to market them better. And of course, other people just wanted to try them out and see if they liked it or how many new tricks it would open up to them, effectively buying into the marketing scheme of "Hey, that freecoaster rider can half cab huuuge stuff! maybe if I got one, I could too!"
Again, this is just speculation & just what I think. That basically no one really started it, but the influx of companies testing new coasters with specs/looks that their riders liked, then putting said riders on the coasters to market them is what started it.
Side note: I remember my friend who rode a freecoaster like 10 years ago was like "I remember I rode a coaster like a decade ago & loved it, but everyone else just talked shit on how gay they were.. Now they're the new cassette apparently" haha