It's essentially the same thing that makes decals silver on a flat paint. On a microscopic level, a flat paint looks like a mountain range, compared to a gloss paint that is much smoother. Think Colorado vs. Kansas. Dirt, oil, gunk, etc. gets stuck in the "valleys", making it dirty and grimy. It also may have to do with the paint itself and what it's made of that makes the grime cling to it. You'll notice that the old gull gray/white schemes were far cleaner. They would get dirty, but it would take much longer than the TPS colors.