because they don't need a catalyst to split the ethanol or other liquid fuel. Catalysts can be pretty expensive depending on the materials involved, so a pure hydrogen fuel cell is less expensive. Straight hydrogen cells are also considered more ecologically pure because the only by-product is water vapor. Fuel cells that use fuels like ethanol release some carbon dioxide along with the water vapor. They're still much cleaner and more efficient than conventional engines but that may not be good enough for the purists. Establishing a large scale ethanol or other alternate fuel infrastructure would also be expensive, but it'd be cheaper and easier to handle than pure hydrogen.