I agree with Eddie that a BIG part of the problem is that, unlike how things were in our youth, people can not drive through a town and see control line activity in vacant lots and at schools, as was so common then. As Brett hints, there seems to be no joy of flight or imagination of things aviation; airplanes have long been common.
I think it is more than that, it is invisible. We fly in a city park in Napa, California. The next park attraction up the road is a skatepark, and around the corner, a BMX track. And many times, both are packed, and cars park on the far side or our circle. There is also a walking trail on two sides. DOZENS of kids walk by every Sunday morning, they never even look up or show the slightest interest. Everyone once in a while, some grandma or grandpa with a baby or toddler wander by on their morning walk, and they will ask about it. But 5-16 years olds, never once. BTW, between the Skatepark and BMX Track - an RC facility. I asked, not one kid has shown any interest in that, either.
I think if we are counting on mere exposure, it's not going to work. I think if anything is going to work, it is *decoupling it* from aviation, and emphasizing the fact that there is some technology and competition involved. It has to be interesting and worthwhile *independent of interest in aviation*.
While I am kind of interested in airplanes, that is a holdover from a brief period in my childhood. Space and rocketry, those were my real interests when I was a kid, and even I would have known or cared nearly nothing about aviation aside from the X-15, and the fact that the astronauts were test pilots. I know about CL only because my dad flew CL as a kid, and seeing various airplanes fly in 1966 (including an unrolling U-Reely hitting some little girl in the head). Later, I couldn't afford RC but my CL stunt career started only once I had gotten a car, and could drive to a site and fly with a stooge - because it was nearly impossible to find people to fly with, or get any supplies, even in the mid-70's.
What interested me was the competition, and the idea that you could practice and get better at it. My position on The Spectrum no doubt aided me, I wanted to do something at a world-class level, and liked the idea of daily training to get it. I didn't know what the hell I was doing nor did I have any help, but being a engineer by nature helped tremendously. I was doing complete patterns with only one lap between maneuvers with a Fox 35/Nobler long before I had ever seen anyone else fly a stunt flight- because I didn't know you had to do two laps. Nor did I know anyone else who knew anything about it.
Point being, what attracted me and still does is the "master all aspects" and "almost anything goes" competition, not that they happen to be airplanes. If I was primarily interested in aviation, I would fly RC, not CL.
I think that is a very narrow niche, and that it will never, ever, appeal to the general public. We seem to have enough people to keep going, with the numbers at various contests more-or-less constant before Wuhan, for several decades. But I am also comfortable that it might all just peter out sometime soon, when people like me age out, or various government agencies finally put an end to it.
150 years ago, horses were transportation and a power source, and every grown man knew all about them. 100 years ago, they were still common. Now, the only people who care about horses are little girls, and many people have never seen one up close, and don't care to. Things change.
Brett