The simple answer to your query is that, even by the onset of World War One, the Gatling gun was a long outdated piece of design and technology. The first truly automatic weapon was designed and patented by Hiram Maxim in 1883 and his name was given to this weapon. By 1886, the weapon was a mainstay of the British Army throughout the colonies and, by the time World War One had began, Mixam had already sold licences to many countries around the world to utilise his design in the production of their own automatic machine guns; The Great War era British Vickers and German MG-08 are adaptions and refinements of the original Maxim design. Whilst the Maxim gun was heavy, bulky and required a team to operate effectively on the battlefield it was water cooled as opposed to air cooled which meant it could readily and reliably maintain its 600 rounds per minute rate of fire. The lack of mobility was also not a huge concern during the Great War which was largely characterised by trench warfare with long stale-mates.

By World War Two, technology had advanced further again where a greater emphasis was placed on mobility. Whilst the Maxim was devastating when used in a stationary, defensive position it simply could not have kept up with the faster paced and more mobile warfare of 1939-1945. Air cooling was given greater attention than it had been before and barrel design improvements along with more efficient heat dissipators increased feasibility on the battlefield. Models such as the MG42 and Bren guns were designed with easily removable barrels for periods of extended fire. These designs were able to incorporate both heavy and light machine gun roles and demanded less crew and equipment to use effectively in battle