It also has reduced color saturation and reduced color accuracy. Second, because most people are apparently practically blind and can't tell the difference between a print from an ISO 100 negative and a print from an ISO 800 negative. So let's assume that you're a little more discriminating than Kodak expects the average person to be. If so, you want slow film. Slow film has finer grain and better color.
Slow film resolves finer resolution than fast film.
At some point, it becomes wasteful to use slow film. If you don't plan to enlarge very much, the fine grain and high resolution of the slow film won't do you any good. If you have a cheap low-resolution zoom lens, then improving film resolution beyond a certain point isn't of any practical value. It turns out that the best modern films are rated between ISO 25 and ISO 50. A typical example is Fuji Velvia, ISO 50 color slide film.