Here's why:

* Coliforms are defined by specific characteristics: They are gram-negative, facultative anaerobic, rod-shaped bacteria that ferment lactose with the production of acid and gas within 48 hours at 35°C.

* Proteus vulgaris does not ferment lactose: It is a non-lactose fermenter, meaning it cannot break down lactose into acid and gas.

While Proteus vulgaris can be found in the same environments as coliforms (like the intestinal tract of humans and animals), it's considered a different type of bacteria due to its inability to ferment lactose.