I wonder if the 19th-century emergence of mass urbanism wasn't a factor. If you're a farmer living in the countryside you already have the space and supplies ne

I wonder if the 19th-century emergence of mass urbanism wasn't a factor. If you're a farmer living in the countryside you already have the space and supplies necessary to keep animals, so keeping a horse along with all the others isn't a problem. But if you're an industrial worker living in a city where space is at a premium, keeping a horse becomes complicated and expensive -- you have to maintain a place to stable it, have to find a way to keep it fed and watered, etc. Even if you could afford all that it's still a pain to deal with, and outside the aristocratic class I doubt most people could afford it. So urbanization meant there was a steadily growing market for some non-horse alternative that fit the facts of urban life better.