I see why. Poop and mucus and blood have very solid and consistent colors, as does urine. Vomit is a mixture of liquid and solid chunks of various colors (no two samples of vomiit look identical), made worse by the unpleasant noises people make when they vomit and the strong smell it leaves behind.
Mucus. You have no idea what sort of bacteria and phagocytes are in that stuff. Vomit may smell worse, but at least the stomach acid kills some of the bacteria, but from a microbial standpoint, it's mucus