Diarrhea is one of the most unfortunate and memorable symptoms of a cholera infection.

We don't have cholera outbreaks any more. That is the true simple answer to your question.

Watery stool is not a major problem. It happens to everyone occasionally. Diarrhea that lasts for a few days is also not a killer or a big deal. A real bad night of drinking or getting unlucky with a meal can cause that and even before modern medicine and sanitation it was just an unpleasant occasional happenstance.

Cholera is an infectious pathogen that contaminates water supplies. When human life commonly got all it's water from the same untreated source, a river or well, then entire communities would get infected and re-infected by the disease. Water has bacteria, everyone is shitting more bacteria constantly, that shit goes immediately into the water supply, water has way more bacteria now. Cycle of illness continues.

Hard to clean yourself constantly, when you can your cleaning yourself with contaminated water and everyone around you is getting sick at the same time. So everything is contaminated. It's unpleasant for everyone but about one in ten infected people are absolutely walloped with constant diarrhea, vomiting, and painful cramps and body pain, all of which will get worse as their body becomes more weak from dehydration and hunger.

Overlay this scenario with other risk factors, say you were in a really concentrated community like New York City, or maybe you are in a really large land army in the field in Poland.

Then you get "diarrhea" that leads to lots of deaths because the outbreak will be to concentrated there won't be enough knowledge and tools to sanitize properly and there are too many people at risk of a serious prolonged case or having other risk factors that make the infection more dangerous.