…is because it's sudden.
All the time you're practicing, you aren't actually riding. You're falling. Then, if you don't give up after all this failure, in a blink, you're riding. No in-between. Failing…riding.
Learning things that are binary like this is quite difficult. They are difficult to market because people don't like to fail. They're difficult to master because people don't like to fall. "You don't get it, but you will," is a hard sell.
Here's a great parenting tip: the best way to teach your kid to ride a bicycle is to wear Rollerblades. I can teach just about any 7 year old to ride a bike in ten minutes using this technique. The reason? For ten minutes, they are riding the bike while I hold them up. Once they get over the speed and steering hump, it's easy. The hard part was the falling.
If your goal is to have a mainstream service or product, then your opportunity is to create non-unicycle moments for your customers, employees and students.