POL (petroleum, oil, lubricants) products are normally stored outside. If the can leaks, you don't want that stuff leaking on your gear or food. Also if your vehicle takes a hit, you don't want all that flammable stuff inside.
Both the 5 gallon water can and 5 gallon fuel can are made of plastic. While both cans are similar in size and shape (so they fit in the same rack), the design is much different between the two. This prevents someone from accidentily pouring water in a gas tank, etc. Water cans used to be black but are now sand (tan). Fuel cans tend to be a brownish OD.