[scott_s]

I'm not saying balance was a problem - quite the opposite. It turns out balance is not a problem. I'm saying that in a world without bicycles, I don't think it's obvious that balance is not a problem. My point is that I find it quite reasonable that people just didn't even think about a two-wheeled vehicle for a long time because it's not obvious that it will be stable.

What people did try to make work was a four-wheeled cart, which I think is in line with a tricycle. I also agree there were other factors - I think there is no single answer to this question, but rather a constellation of factors. My claim is that I believe it reasonable to include people's lack of imagination that two-wheels could work in that list of factors.