Most riders are not familiar with the sport.
Yea, they may have some knowledge, or seen a demonstration, but hardly anyone knows what's involved. Very, very few have ridden a bike and not many understand what's involved in starting out or how it could benefit their other riding.
Seeing a Red Bull video, while impressive does not draw people in to personally participate because it seems out of reach.
Balancing while standing still is intimidating, and most don't know that it's possible for them to pursue this.
There is very limited information online as to how to get started. (Ryan Young on YouTube is good though...)
Bike demos, basic intro, basic skills, more promo of benefits of trials on other types of riding, more event promotion, better online content will all help.
Given the small trials community, it's a tough problem. Significant growth will require lots of effort from manufacturer/ dealers/ clubs/ riders in a coordinated way. Better online content is needed.
The manufacturers will probably need to work together to make it happen.
Not easy.