Because cholera existed before the 19th century, and people didn't understand that contaminated water was the cause?

So as late as the 19th century, people were just drinking regular water, believing there was nothing wrong with it. (This, plus the fountains and wells that have existed since antiquity as public places to both drink water and wash your hands... eww)

Cholera only existed in the old world and is a byproduct of animal domestication and urbanization[1].

[1]: https://www.youtube.com/watch?v=JEYh5WACqEk