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.

I don't know how prevalent tricycles were but in the article the advertisement for the "saftey bike" says "safer then any tricycles" so there were some. Not sure why they were safer then tricycles of the time. Were these tricycles tall?

Found one picture https://www.google.com/search?q=1860+tricycles&oq=1860+tricy...

I kind of wonder what it would be like to build a tricycle using pre-1800s technology. What would be the most difficult challenge? Lack of chain drive? Lack of quality bearings? Durability? Materials? Lack of rubber?

I bet a bicycle using 1700s tech would be horribly expensive considering the GDP per capita too. Not having rubber tires would make them dangerous as heck too. Traction is a huge factor in vehicles that aren't animal drawn. A horse/ox/human provides a ton of traction and stability to a vehicle when they pull it.