That's because wild cats get a lot of their water from their food, and they don't like drinking from stagnant water. Most people only feed their cats dry food and use a water bowl.
When I first got a water fountain for mine, I had to start cleaning the litter box twice as often. They also won't drink out of it if the water isn't running.
They're finely tuned instruments. High octane race cars would be an appropriate example. The majority of those end up "dead" from destroying their engines.