In a word ... nostalgia. I guess its like an old 1950's or 1960's VW Bug. They still have aficionados.

The J3 was the poor man's Stearman or WACO. Cheap to buy, cheap to run, land/takeoff almost anywhere, great slow/low flyer especially for cattleman or ranchers in West Texas where I grew up.

In the RC world, there is more to the Cub series than meets the eye. They are not as simple to fly as you would think. The takeoffs can be interesting at times, once in the air if you fly scale-like and not over powered, the plane crabs in heavy air. I've own 2 and loved them both. My 1/4 scale H9 Super Cub (really modified to be a PA-11) is still lots of fun.