I'm noticing that the "slow" films such as 100 and 200 ASA are becoming harder to find, and I think I know why: the average consumer doesn't see any benefit in shooting slower films. So if there is no cost advantage (increasingly the case), why not "upgrade?" So consumers buy less slow film and it becomes less available.
But caveat emptor! As is often is the case there are tradeoffs! Compare a developed 100 or 200 ASA negative to a 400 or 800 ASA negative on a light box. You will notice the former has greater "density." I.e., the darkest parts of the negative are darker than the darkest parts of the latter. This makes sense since faster emulsions are faster partly because they are thinner in order respond to less light.
But, fewer photons means less time for the film to capture subtle shades of dark and light. This I believe is why images captured on fast film often lack the depth of those captured on slower film. Especially if there is a large range of dark and light in the image, a fast exposure tends to compress these into fewer tonal values. The result, in my experience, is that images from faster film look flatter (this of course is in addition to the issue of grain being more apparent in faster films).
So I challenge you: try some slow film next time and see the difference!