The difficult part is not balance but looking ahead. When you are first learning to use the pedals, you instinctively LOOK at them. That's what makes it difficult. I learned on a standard bike but did not pedal until somone encouraged me to try. I was going downhill fast and everything without using the pedals. :)
Your bike is upright and the handlebars straight. You add velocity to it. There's no tipping there. It will go straight forever. It's a self balancing system - unlike skiis. You only tip the bike over to turn and that (counter steering) only after a certain speed.
I maintain that it's looking at the pedals. Looking itself doesn't do anything but a newbie will move the handlebar or lean while looking and or won't generate enough speed. Couple of days on the saddle and they can look anywhere while riding.
I disagree; you don't balance on a bike, you adjust the balance point under the bike as it tips to one side and the other with pressure. This is why beginners "S" their way at the start, then get finer grain control. This is similar to skiing, where you don't balance on top but keep pressure applied to an edge.