This may have been already mentioned but sometimes things lose popularity just because they lost popularity. Model airplane flying is a very social hobby. At my local RC club, you will see a bunch of "regulars" who seem to always be there, even though they don't bring anything to fly. CL and freeflight seem to have reginal popularity where it is still popular because it is still popular.
When people start dropping out of a scene, a tipping point can be reached and nobody shows up because nobody shows up. People get disgusted when they travel across a couple of states to enter a contest and only a couple of other competitors show up and so they drop out of the scene themselves, sort of a reverse snowball effect.
I used to race Hobie Catamaran sailboats and at a regatta there would be up to 200 boats showing up during the peak of the boat's popularity and then it collapsed in just a few years for the same reason. I found myself driving hundreds of miles to a regatta only to have one other competitor sailing in my class. It didn't take long for me to lose interest also.